After giving it some thought, I have come up with a potential, albeit extremely unlikely explanation. Assuming the ref spotted the ball for the previous set of downs, he may have had a good idea of where UM needed to go in order to get the first down without the benefit of the chains. The chains are, I'm sorry to say, a very inexact science. Everything is based on where the side judge places his back foot at the beginning of the series. If he is a couple inches off, then he's a couple inches off. The stick might have been placed a couple inches off, which could have exaggerated the problem further. Finally, the placement of the clip could have been a chain link or two off. Any one of these things, alone, wouldn't be that much of a problem and would never be noticed. In concert, they could be enough for the ref to notice a difference and overrule the official measurement. As I stated, it's extremely unlikely, but it's the only explanation I can come up with at the moment.