On the US flights over holidays seems they are overbooking Economy a lot. So if your flight is 0 in all classes, given the time of the year, they are probably planning on upgrading a lot of pax.
For the asian destinations you might see the opposite - Business is very often full but lotīs of seats open in Economy. I have been asked if I am willing to fly in Economy when trying to change date for the return flight from Asia (holding c-ticket).
Seat guarantee for elites is also a factor, I guess mainly on the asian routes.
So itīs a tricky game for rev management for sure.