Well, obviously not everyone called it that and several other cartridges, the .30-40 Krag did not get called that until later. I wonder what the soldiers there were issued .45 Colt cartridges called them? If they originally called it the .45 Long Colt, I don't see how it could have become incorrect later, though that sort of thing does happen.
Sometimes incorrect names get carried over from an earlier name and also, designations sometimes are applied much later on to something that never officially carried that name to begin with. Collectors are prone to doing that sort of thing, although it does help sometimes. For instance, the first "field jacket" issued to the army in 1941 was called simply a "field jacket." It wasn't a Model 1941, though it was sometimes called a Parsons jacket. The garment that replaced it was officially a "coat" but ever since then, everyone has called it a "field jacket." In fact, the term gained enough currency for the British to rename their DPM smock a "field jacket," which is so marked on the label.
In like manner, you may notice that even then the term "ball" cartridge was already in use, though obviously the bullet was not a ball but a conical "bullet" shape, which you may also notice already had a flat point that .45 Colt bullet retains to this day.
And did you ever notice how cartridges from that period tended to come in very small packages?
