I had a similar experience on UA110 a month ago or so and was able to upgrade from a P fare with miles and co-pay. I was able to get the upgrade confirmed at the airport but had to ask for it at check-in, when they weren't even to give me a confirmed seat assignment until the gate (but I got the seat I asked for at check-in). The seat map prior to arriving at airport showed no F-class seats assigned, so I am guessing the rest went to non-revs. As someone else said, perhaps there are frequent aircraft swaps when they don't like sell the seats, but the the OP said some seats were confirmed in F class in advance. So not sure what the real deal is here.

Adam