Charlie Z.
In looking at that data you mention, considering the loads listed here as being hotter than those of the .375 Winchester, you need to look at something that many people miss.
First is the difference in brass length, and second, most importantly, the thinner case walls on the .38-55 brass when compared to those of the .375 Winchester!
In the .375 Winchester I can gain almost 8% case capacity just by switching from factory .375 Win Cases to fire-formed .30-30 brass! The same concept apples to the .38-55 when comparing it to the .375 Winchester, the .38-55 has more available volume than that of the .375 Winchester, hence more powder, and comparable or lower pressures in some situations.
None-the-less the .38-55 is a potent customer when loaded to its potential. It is a very inherently accurate cartridge as well!
Hopes this helps to clear things for you!
God Bless,
Marshall