[thr0w__4w4y]

Your comment is warranted, but I think I know at least one contributing factor (normally fly around 200K / yr on United)

I'm making up an extreme case to make the point, but let's say the plane's business / first cabin is 100% empty / unbooked, and the economy cabin is full (even overbooked) and the standby list is 30 deep.

Even though UA is reluctant to give out free INT upgrades, if it means they can put 30 more people on that plane and collect the revenue, they'll do it. There's more to it than that, I'm sure there's a heuristic, in fact one day I hope there will be an AMA "I code the conditional logic for airline XYZ's algorithm <for Global Services> <for INT upgrades> <for domestic upgrades>" but until that time, we can only discuss what we've observed and speculate on the rationale.

Me, I'm still wondering why when I'm first on the ugprade list and there are 8 unsold first class seats, why do I have to board (in my economy seat), settle in, unpack and decompress before some gate agent comes waddling down the aisle 30 seconds before the door closes to tell me I've been upgraded. It was clear 30 minutes ago that was the case. First world problems, I know.