Well, I started hitting walls with 20 reps for arms. It just is hard to get to 20 at the limits of what my arms can do, but using rep/weight calculations of the higher amount for 15 reps, I am able to achieve and upgrade my weights, even when I have been stuck for weeks at the limits of what I can do at 20.

This only seems to apply to arms though. Legs and back seem to adapt just fine to upping weight at 20 reps, and haven't had problems there. My theory is that large muscles have a higher ability for long term stress. The little biceps and triceps just can't keep going.