They're coming from the normal range of performance you see from almost anything that is manufactured with less than rigid quality control. There seems to be a significant variance in how the locked-in needle valve limiter is set by the mfg. There seems to be quite a variance in almost all ARFs' wood selection, and therefore the balance and strength of each one. There seems to be differences in the power output of the engines.

For example, your "one very minor one". If every other owner doesn't see it, how did it happen to your example? Same way others saw something you didn't.