100 fps?! why would you ever set it to that? anything over 50 is a waste of cpu/gpu... and its a good idea to leave some cpu left over when you need it

as for the fps not dropping.. .that's due to blizzard not recognizing that it has lost focus, and dropping to the lower limit. moving your mouse in and out of the window (if you have focus follow mouse checked) will drop it to maxfpsbk

hopefully on the next release it will behave better