1. Humans actually have only four fingers, considering that the thumb is not a finger.
2. Animators typically give characters 3 fingers or less (and a thumb) due to the fact that it is quite impractical to put more detail and effort into something that, in the end, does not matter all that much.
3. It does not matter all that much because the animators in question can do whatever the heck they want. For all I care, all of the characters could have like mitten hands, where they have like one huge finger and an opposable thumb. As long as the animation is cool, and has a dope soundtrack over it, I could not care less.