if you have focus-follows-mouse enabled, then if your mouse hovers over a wow window.. THAT region will become active.. driving its fps up to maxfps and dropping the other windows fps down to maxfpsbk

edit:
also, 20/60 might be high. you want to find the point where all your alts get the maxfpsbk while in the background .. and hit the maxfps when active. anything less means you are pushing your hardware too much and should consider backing down your settings. (using lower details also helps)