I noticed the same, so I took a look at the source code. It's not bugged and it's not random either. Sadly, it doesn't become clear during the game that it's not random. You have to do specific things to get other frogs when they're still small, or it will resort to the normal frog: Feed them 4 fireflies or feed them 2 fireflies and make them go swimming two times. Then depending on which frog you want to achieve, you have to do specific things again.
So, try lots of stuff. Feed them one kind of bug a lot and see if it changes their appearance when growing. Things like that.