Because people do all sorts of crazy things when feeding horses.
It's generally best to follow the directions on packaged feeds unless you want to do all sorts of complicated math. Unfortunately some people think that by adding "just a tich" of several different feeds they're doing something good for the horse. Usually they're just causing problems UNLESS they are taking the time to actually balance the ration.
Personally I try to follow the "keep it simple stupid" rule when feeding and my horses (including my 30 yr old) look great.