Uptown, it's because the Supercharger hadn't changed speeds, so when your at max throttle at those speeds, the supercharger is already working as hard as it can to reach the manifold pressure requested. Once you hit that point where WEP comes back, that's when the supercharger changed speeds. At least that's my understanding.
I'm guessing it varies very much from pilot to pilot and scenario to scenario, I watched something that said WEP was used in all sorts of maneuvers among spit pilots. I assume it was probably used more in 1v1 scenarios than anything. I know this is considered somewhat of a lame source, but I was watching a Dogfights episode where they specifically said a P-47 pilot threw the Jug into WEP when engaged in a continuous flat turn with a 109, and actually started gaining on the 109.