1) The other issue is, it could measure the volume, tap it on the bench and measure it again and shake it up and measure it a third time and most likely get 3 different volumes.
2) From experience and discussions, I have never really though as volume of any powder as being fixed.
3) But if I was really going to get down to the biggest reason I trust weight compared to volume. I can use a check weight or zero my scale. I don't have a way I trust to verify volume, other than to weigh it.
4) To get somewhat scientifical, powder burning is a chemical reaction. Density of powder can change as noted above. The mass of the material has a much closer relationship to how much material is available for the reaction and for all intents and purposes is fixed for a given 1 or 8 lb can of powder. I do not remember any calculations off the top of my head for any change of state of a material that uses volume for the amount of material being reacted (it has been a long time for in depth chemistry).
👆This is why I asked, you are confusing some things.
1) If you have no way of confirming volume, then it isn't "most likely" or unlikely to get 3 different volumes; the probability is unknown. If you saw a difference, that was in the number of kernels, or the density of the pack. neither of which have anything to do with the consistency of the volume. Shaking and tapping doesn't change the volume of a volume device(assume it's not made of tinfoil). If you shook and tapped on your scale while trying to weigh, would it change the weight?

2 & 3) That's because people rarely understand volume, and it's really the crux of the matter.
Volume is the area available to put something in. It isn't the weight in the area, it isn't the number of kernels in the area, and it isn't how densely packed things are. It's just the area. If you got an empty amazon box that measured exactly 2 cubic feet internally. Then sprinkled in packing peanuts until it was full and level, and happened to count that that was 100 peanuts. Then if you took those 100 peanuts out and squished them and repeated the sprinkling. You may find that in the end it now holds 200 peanuts. You never changed the volume of the box, so the number of the peanuts is irrelevant to the boxes volume. Similarly whether or not the weight changed or the density of the product found within the volume, is also irrelevant to the volume of the box.
So for the consistency piece, and the point I believe you were chasing.
If you were trying to accurately weigh 55gr of powder, would you: Weigh it on an unlevel table. Tap on the ballance beam randomly while trying to take a reading. Turn on a leaf blower aimed at the scale. While doing all of those things, you decided that you had the correct weight you wanted. To be sure, would you then dump it into a volume device like a teaspoon to verify the weight; which your uncle told you held exactly 55gr? The answer is quite clearly, of course not. So if you wouldn't do those obviously ridiculous things for accurate and verified weight, then why in the world would you do such things to a volume device to judge it?
4) Yes, but no. The thing most folks forget or don't know, is that the density of powder is never a fixed thing(talking extruded specifically). The people who design and manufacture powder, do so with volume in mind. The bulk density, which is the volume, for a given weight; is constantly changing. If you take a couple kernels their weight will vary, as will their volume and porosity; which is by design to control the burning rate. The more an individual kernel weighs(nitroglycerin content), the longer and more porosity it will have; with the inverse also being true. This can be seen in many applications. As Mike said, some of the most accurate factory ammo, has rather large weight fluctuations; because they don't load by weight. Similarly if you read some of the ad fluff on Nosler ammo, they tell you that they very carefully and precisely weigh each and every charge when loading. Uh-Huh... Go take the factory tour, they load by volume; the same as every other manufacturer that puts out more than 20 boxes per month.
So to try and circle the wagons back around to the point I believe you were trying to get at, which is consistency; how do we accomplish that?
I agree that without a reference, you can't know how accurate you are in either system(weight or volume). That is a perfectly reasonable, and well thought concern. Short of another calibrated volume device, we honestly can't know how well it works. I would posit, that not that many folks actually regularly calibrate a scale; especially when starting out. I know I didn't for many years. However what most people do when trying volume, is use what is only a reference, as a fixed unchanging thing to determine another system. So when they realize that the two systems don't reliably interchange, because they don't really understand volume; they tend to default to thinking that the volume system is faulty and unreliable. Of course this is a mistaken line of reasoning, and ignores some very basic principles of a handloader.
For example:
If you open the Lee manual, or the data on their dies, or look-up the data on their website. You can easily still find VMD's for just about any powder that exists. However, just like the data found in any other source; the actual important data is missing. No one lists a lot number, a date tested, whether or not Hodgy or anyone else has silently swapped suppliers, etc. So the prudent reloader will simply determine the VMD of
HIS bottle of powder. He will remember that the VMD is a
refernce point only, it is not a direct translation, to data that is listed in grains. At that point, he would discover what everyone who handloads has discovered(regardless of the system used). That their gun essentially never produces EXACTLY the same velocities per charge, as listed in the load data. They know that because of the unlisted lots and dates, and how powder ages, that their lot and gun is simply different than what is shown in the data.
So if he were a weight guy, he would test and watch working up; and stop when the evidence tells him to. Volume would be no different. If his
REFERENCE for the load range listed in grains, into 3.00cc - 3.65cc. He knows that in fact, he may likely not be able to precisely do such a thing. And just like when weighing, he will take caution to be as consistent as he can. Meaning, if he is OCD and tapper, then he should always tap the same amount, places and times while laoding. If he does not, then he shouldn't randomly start doing it.
In the end, if he doesn't think that volume is his cup O tea; that's perfectly fine. For me personally, I found that my group sizes and ability to shoot at extreme distances became much easier when I changed to volume. Again personally, I found that less meticulous care in the loading process; proved out to have less impact on groupings; compared to when I was weighing charges.
That may not hold for everyone, but has been my experience.
Cheers