If you have prolonged zone time on the PP, which will as you correctly state increases the probability of success, then you will score more often before the 2:00 expires, which will be reflected in "per 60," since you scored in a hypothetical 1:30.
If you're "controlling well but not scoring," then there's no difference shown between that and "controlling poorly and not scoring." The stat itself is not flawed if it doesn't explain "why" a PP is failing.