I believe the only logical answer is that different people are evaluating. If it is the same people evaluating then they changed their mind. It is obviously difficult to rate borderline degrees of cancer cells. The biopsy is just a snapshot of specific areas, while the pathology is looking at the whole thing. You are correct in that if there was any 4 rated gleason it should have been a 3x4 on the pathology. In fact most of the time the pathology, if it is different, is usually worse for that very reason.
The good news is that your pathology was an improvement, and that is a blessing.