Many factors:

1. The people ahead of you could have seat preferences (no window, no aisle, and/or no middle) set so they'd get passed over if there's no seat that matches their preference.
2. There could have been only one seat open for the upgrade bucket (at a time) but they have more than one person per PNR. The system won't automatically split them.
3. They could have SDC'ed or changed onto the flight after you cleared.
4. They're could be on a partner ticket, so they don't appear on the upgrade list until they've checked in, after you've cleared