I'm guessing a rounding issue based on energy build from damage incursion.
THEORETICALLY lower defense IV means more damage taken. More damage taken leads to faster energy gain, which leads to more Dynamic Punches thrown. In practice this difference is completely negligible and would only be useful for extremely case-specific scenarios, none of which I can even think of for Machamp.
So far as I can tell these two would perform identically outside of specific defensive breakpoints/bulkpoints. Lucky and time of catch make zero difference on damage output.