Having died at #1 on the list more times than I can count, even with seats available - and also the occasional upgrade once in my seat with First being long since sold out, one of the things that I am realizing is there is a lot of movement of upgrades and WFBF (J/F and buy-ups) at the last minute that take some of these spots.... if a J/PZ/whatever is on a flight and there is an IRROPS or need for a change to another flight, UA tries to keep you in that class of cabin...heck, if you PZ/PN up days in advanced and then there is a problem at the last minute, you might be able to get the last J/F seat on a much busier option..... the actual impact of this on the UG list is unknown but probably significant...