The voodoo that Qantas does so well. In 5 years i've had around 200 odd flights and not one single opup. I know NB's that have have several and have probably only had 20 flights in the past 5 years, all of them on routes i have flown on regularly and nadda for me.
The only real reason I can think of for the upgrades being lucky is as some have mentiooned, moving you to seat a group together. It seems the only constant in the equation cause lets face it, groups travel often and don't want to be broken up.
Drew