[elhigh]

I would guess there are a few sliding scales of drag and efficiency going on inside the engine, and you have two different scales experiencing their best values at those two speeds in that gear.

I can't remember where I read it but I've seen it stated that ~1200fpm is the optimum cylinder travel speed inside an engine (can't find the resource now), but maybe you're getting better airflow at the other speed - whichever one it is. Different factors combine here and there, sometimes to add to each other, or cancel out or subtract from each other.

And those units are inches of mercury.