Just a quick question about how UA inventory works.

I have access to Apollo and have been tracking inventory on a international flight for a couple of days, this afternoon it showed the following:

F9 J6 C3 Y0

On the seat map there were 9 F seats, 6 J seats, and 11 Y seats (which were all E+). I assume that the back is oversold and some do not have seat assignments so that is why there are seats on the map in Y.

However, a few minutes ago the inventory went to :

F0 J3 C3 Y0

The exact same seats are still on the seat map 9 in F, 6 in J, and 11 in Y.

I assumed they were upgrading people from Y to J and/or J to F, but I don't understand why the seats were not assigned and are still on the seat map?
When upgrades are processed, do they not assign seats?

Any insight into this would be greatly appreciated.