Because no matter how short or long your draw length - a shorter brace bow still releases the arrow closer to the rest than a longer brace bow (one of the reasons an over draw is harder to shoot)

The closer an arrow is release to its pivot point (rest) the more exagerated any deviations form center have on the arrow flight.

this is one reason it is believed drop aways may lessen the effect of a short brace (because that "pivot" is gone)

I also agree with Ossage in that the % difference is not the same

With that said - short brace bows can still be shot rather well, and it depends on your form and how "well' you want to shoot.

I don't care what your DL is a 5-1/2 Brace is going to much harder to hammer a 60x game than a 8" brace - because the margin of error is so small.

Now throw in a broadhead and the forgiveness factor of a short brace can really expose bad form.

Then add the clothing factor and problems can mount.

But if your good - you can shoot em pretty darn well.