Hate to break this to all, but they're pretty random, and ther result of marketing decisions. May vary a bit from company to company, but that's the case for most that I'm familiar with. Hodgdon's "H380" came from Bruce Hodgdon's favorite load with that powder (GI surplus at that time), 38.0 grains in his 22-250. I understand that IMR3031 came from its ability in replacing cordite grain for grain in charge weight in the British service cartridge; hence we have IMR "303"1, or IMR 3031. There's more, but you get the idea.
Pretty convoluted, for the most part, and a good reason not to try to read too much into a designation.
Kevin Thomas
Lapua USA