1991 Upper Deck Football, as well as baseball, had some variations as far as the hologram on the reverse of the card.
I recently added two of these variant cards to the Herschel Hoard.
I'll use the 1991 Upper Deck #99 card as an example. The front is the same on all versions.
What follows is the most common reverse:
There is also a version which has a 1990 hologram. I have been unable to locate any of these as yet. Here is a close up of that hologram: