Forget the pulley, that's not it. There was nothing wrong with the replacement pulley set you bought.

Shimano has this thing they call a Centeron-G pulley; the upper pulley is slightly narrower than its bushing, allowing some lateral float (if there's an actual diameter discrepancy, you're looking at wear).

The purpose of this is to allow for poor adjustment. It actually detract slightly from shift quality; some folks (myself included) are apt to intentionally swap the pulleys.