That isn't a "problem". The reason the fitness sliders return to normal ranges is that there HAS to be SOME kind of limit, otherwise the sim would turn into a giant blob of either fat, muscle, or both, as they would never stop increasing. Since NORMAL people do not end up looking like that without artificial methods not present in the game, it is reasonable that those are the set limits.
Is there any way of increasing those limits? Not by a big amount, but enough for some more diversity. Or are those limits hard-coded into the game so that changing them is simply too hard?