It's called "Artists Are Not Architects". Basically, animators can't be assed to give a shit about proportions or heught making any sort of sense in anime. Same reasons most girls bust sizes can change dramatically depending on the scene, A-cup to E-cup, that kinda thing.
For a good example, see, say, Ed Elric's listed height of a bit under 5 feet as opposed to how high he actually is compared to other characters. Makes most of the people in the FMA world something like 8-9 feet tall in terms of numbers. Or that one chick in GANTZ who suddenly actually becomes 9 feet tall onscreen because the animators weren't paying much attention.
Makes things more interesting.