This may muddle the discussion a little

Oldgold is correct about the standby list being cleared by elite level. But there is more to it.

When the A seats are released, day of the flight, If seats are available, it is first come first served. So a non-elite with a prestige pack certificate can get seat if they are early and a platinum checking in a few minutes later doesnt get the seat.

Once they start building a standby upgrade list, it is by elite level, then by checkin time. I remember one flight last year, that I checked in about 20 mins before the flight and got the one standby upgrade, I guess I was the only gold on the list.

As for the gate agents "changing things". This can and does happen. So always be nice to them