I almost missed that these cards were different and nearly threw one in to the duplicate stack. If I had come across them separately and not in the same stack, I probably would have.
The card on the left is the 2005 Upper Deck base card in the Pennant Race subset. The card on the right is the 2005 Upper Deck Retro card. It seems that one box in every 12-box hobby case of 2005 Upper Deck Series One was filled with the Retro cards.
Despite not noticing initially, there are actually quite a few differences on the front. The left and right borders on the Retro card are white. The background has been tinted blue and some vertical stripes have been added behind Prior's head. The team name is also now in a foil band instead of just hovering over the Pennant Race banner.
The backs however are a different story. They are identical.
As someone who didn't collect when this set was released, it would have been appreciated to have the name of the parallel set somewhere on it. Maybe it was more widely known in 2005 when the set was released but I had a hard time finding information about it when I didn't even know what I was looking for. Ok, not really a hard time but more time than it should have.