Have you ever noticed this about MMA fighters?

I think a lot of that has to do with finding the best sparring partners. You might want to roll with one guy because he is really good at submissions, but roll with another guy because he is great at defending takedowns. You might want to box a boxer to improve your hands, but train with a muay thai fighter to develop knee, leg, and elbow attacks. Some gyms are fortunate enough to have experts in every field in them, and some MMA clubs are not.