Creatures in Tibia are clustered in different levels of Dificulty
I've heard that the bestiary is programmed to always have 1 creature from each level.
That tries to keep the preys "balanced" for every player. I'm not sure on how many dificult levels there are (I would guess 9) but I do know that is not exactly like the bestiary: easy, hard, etc... because I've had 2 hard creatures in the same prey many times.
I can't find a source for this and I'm not 100% sure, but I've heard that many times while reading forums, etc... And from my experience, it seems to be true.