From what I've seen over the years, some of it is route specific. For example, when I was at LGW, the Club cabin on the ATL service was quite often at least half full of involuntary upgrades. There was no pattern as to who was upgraded, the decision was often made prior to check-in, so it was not always possible to chose who to upgrade according to how they were dressed or how nice they were. There weren't too many Exec Club members in Y to upgrade either. KIN was often the same, with Y being heavily oversold - but the flight would still go out with empty seats.