When we mapped our turbo car, years ago, I learned about reduced cylinder fill as the rpm got up high. You just dont have enough time to actually fill the cylinder, like you would at mid-range, and what you get is later. That means you are going to get a smaller bang, so to speak. If you picture the time event, over the range of piston position where "pushing" on it does any good, you see that the smaller bang needs to back up in time, a little bit, to get the largest proportion of "push" on the "sweet spot" of the power stroke. It really is milliseconds of difference, but that is quite a few degrees at high rpm.
That "less air" business meant that my injector duration pretty well flattened out above 6000-6500 rpm....and I may have left it a little rich. Unfortunately, my crank trigger mount broke, the engine banged the turbo (hard enough to break bearings) and the exhaust blew some chunks out of the dyno bay concrete floor. I wasnt invited back to play with their big toy.
Variable valve timing race engines sometimes have two places where the intake cam is fully retarded....idle, and anything from somewhere past the torque peak all the way to rev limit. Anywhere else, you are about as well off to just go to full camshaft advance and tune with fuel and spark. Trying to fish around with interval hold positions just makes stuff wear out, anyway...except maybe Suzuki's ramped lobe system. Thats pretty cool but I dont know if it is used, yet. Probably all kinds of stuff I never heard of, going on by now.
Anyway, knowing that variable intake cam timing gets retarded for high rpm, kinda explains why spark needs to back up as well, dont you think?
