I don't think this is always true. My city measures buses as being on time at inner stops as being between something like 5 minutes early and 1 minute late (yes, you read that right). Timing stops, where they have to wait, are pretty infrequent (mostly bus terminals).

I don't know how common it is but it does exist. And buses perpetually being early means that if you're on time you wait even longer for the next one.