Because: When Buffon reach the ball the modena player was in the perfect position. However Bupon tip the ball to upper goal pole then when the ball bounced back off the pole the modena player was already on the offside position.
IF: Buffon tip the ball w/o hitting the pole the goal would be allowed. And the Modena player will always be onside position. Just like the 3rd JUVE goal (Treze shot the ball but hit the keeper then the ball bounced back then he hit once more).
++ [ originally posted by -S- ] ++
Yeah, I realised he was onside after watching it for the second time. I didn't see it clearly at first how the ball got to his feet.