Well, we just use our creative spark to draw something we are excited of :) We want to draw modern tiles - we draw modern tiles. We want to draw some strange creatures - we draw them. Sometimes it is just an unnamed thing that looks cool.
But don't worry, I am working now on the next gargantuan update that will contain a description+tags for each tile. I am sure everybody will be shocked!
About tiles:
A1-A14 - Various animalfolks (but A13 was designed as gorilla)
A15-A31 - More or less all your assumptions are right
A17 - Old Rus strelets (a warriror with halberd-rifle)
A22-23 - Some creepy faceless guys
A28-31 - Partially outdated tiles after last megaupdate with 2000+ creatures
B1-B12 - Various creepy monsters for horror games
C1-C13 - Various non-humanoid creatures (C13 - Dune-like wurm)