My guess is the flight is overbooked because it's a holiday week. CO blocked the F seats because they will need to move Y pax to F (either elites or op-up, but most likely elites).

The "tens of dollars" upgrade is more than likely the fare difference from the fare you paid to M, B or Y-UP and is CLEARLY described as such on the fare buy up page. Would the OP care to comment on the fare paid and fare class?