I asked Robert Glock once, he said they just started numbering them sequentially after the Glock 18. I'm not sure why that was relevant, probably the patent numbers as stated previously.
I had an ancient Glock brochure once (first one they ever did), and it was something like "ideal for police, sport and military" and the idea was the Glock 17 was for the police, the 17L was for sport and the 18 was for military use.
But hey, we could always just give them our own names... hmm... can you guess which models I'm talking about...
"The original"
"The smaller than the original one"
"The one that goes b-b-b-b-bang"
"The one that Americans wanted that is too big for anyone's hands"
"The long one"
"The slightly shorter than the long one so it would fit in the IPSC box"
"The chopped one that is still too big to fit in anyone's hand"
"The thinner chopped one that we finally perfected"
"The one with the bigger hole in the barrel that we copied off S&W"
"The one with the even bigger hole in the barrel that everyone is still wondering why we made it"
"The one that we copied off SIG Arms"
"The one we made because Clinton banned large capacity magazines"
Etc.