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The spark advance for best power (MBT) as RPM rises is determined by how fast the time available for combustion decreases vs. how much the ever-increasing turbulence speeds the burn. Typically from mid-RPMs up, the two are pretty much balanced, giving a fixed advance requirement.
On the detonation point, the missing factor is TIME. Once all the conditions of mixing, pressure and temperature have been met for detonation to occur, it DOESN'T! At least not for a short but critical delay period. If during this interval (which of course encompasses more crank degrees as the RPM increases) the normal flame front arrives in the subject region, no detonation occurs. For this reason, detonation is much more prevalent at low RPM.
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