Difference in RPM will cause a difference in crank flex,,,also,,,every moving part of an engine will change tolerance with either RPM or Heat,,,,,I know it dont seem possible when you first look at it,,but metal,,,any metal will stretch and deform with enough reciprocal, or centrifical force.
We are only talking mere thousands of an inch here,,,,,piston travell and ring run in,,will cause ridges to build up at the limmit of the TDC point of the cylinder,,,when the RPM is increased all of a sudden after years of consistent low RPM,,,those thousands of an inch will give you lots of greif.