For example, some fare classes on United flights can fetch 300 % of miles flown as PQMs while the percentage is generally capped at 150 % for the same fare class on a partner. (valuepenguin.com)
Now if there are multiple 75Ks on the waitlist that are booked in the same fare class, then the tiebreaker is based on when the ticket was booked (oldest booking has the priority on the upgrade list). (singleflyer.com)
Some airlines have procedures to rebook you automatically by forcing the same fare class on their own flights (Delta, for example)-- earnings would then not change. (rewardexpert.com)