I recently flew this route and cleared at the gate both ways. Day before flight was C9 Z9 and Y5 to Y1. Day of was down to only a few seats showing open on the seat map. Got a middle seat one way in C and an aisle the other. Again both didn't clear until just before departure. If I remember right I was traveling on a Friday and Tuesday.

My guess is they are selling upgrades at check in to non status passengers for $$$ before 1k's on SWU's are cleared. Just my guess though based on the seat map and united.com showing fares for purchase.