I say it is loading and unloading of the prop in flight. The 200 rpm adjustment from peak is a good safety measure for avioding a lean run. When you are setting hi-speed needle you are putting max load on the engine because airplane is not moving. This is where you would be concerned about a lean run. I would take a guess and say at straight and level flight, the engine would have the lightest loading and therefore would actually be on the rich side.