I would say MaxPass FREQUENTLY gets earlier times in my experince, more often than not. I wouldn’t call it occasionally. That’s why it seems qeuestionable that there aren’t extras for MaxPass users.
It is not completely impossible that Disney adds to the pool based on crowd levels, additional tracks/vehicles added, other rides breaking down, anything that could result in a change to the anticipated ride capacity. FP machines would not go backwards in time, so this would only benefit MaxPass users.