That's system-wide load factor. Either US has a bunch of spare Airbuses lying around to plug into flights, or they have to swap aircraft. If you replace a 321 with a 319, you need to put the 321 somewhere. If you put the 321 on the 319 route, you have a bunch of empty seats, since you sold tickets based on the 319's capacity.

As far as yield goes, RASM is increasing.