R inventory is in flux. They make changes to that number often. I've seen R availability then a few hours later it could be gone if they so chose to pull it out of that inventory or someone with miles Or RPU can "purchase" the upgrade.

The old CO would have different (and unknown) criteria for each flight, if I'm flying BOS-EWR at 6am Saturday morning as a top tier, I'd usually get a 5 day upgrade.... But for the return day 5pm on a Monday, may not get that until the gate if I'm lucky.

Also say UA opens some R at say 10:00am and the flight time is say 1pm two days later.

If the sweep runs t-minus two days to the flight at 1pm and R is a available then the next elite in line will get the upgrade, could be you or a GS or a 1K with a E fare for example.