I've had very similar incidents -- where .bomb seemed to "predict" the imminent UPG availability. In fact, I used to see it frequently 2-5 days before my transpac flights... I'd call to ask about it, they'd say the UPG availability wasn't there, and then a couple hours later, I'd get the upgrade that'd showed as being available.

But I've also recently had an odd experience which tells me that UA's UPG system doesn't work flawlessly (coming from the database world, I'd say that there's a bug with a database trigger somewhere, but that it's on the back end and not with .bomb), and that the "it's a .bomb error" crowd isn't always right (though they may be sometimes/frequently). Here's that experience:

Last Wednesday I booked a B fare for myself BKK/NRT/SFO/BOS on July 1 (then EWR/SFO/LAX LAX/SFO/NRT/BKK, but those parts aren't relevant). I applied an SWU to the BKK/NRT/SFO/BOS piece (all 1 day of travel) and none of it cleared.

Then Friday I booked a Q fare -- same flight segments -- for my wife. Her SFO/BOS segment cleared immediately when I applied the SWU to her itinerary. Mine still hadn't cleared.

My wife has MP status of "general member." I'm Global Services.

After that, I called the GS number for something else -- I wanted to apply CR1s to the EWR/SFO/LAX piece of our itineraries, and had them in my account, but .bomb would only give me the choice of using mileage or SWUs to upgrade. When the agent applied the CR1s, she said she'd forward my itinerary to the Inventory Management desk (so that if the loads are low enough, they can clear my upgrades early). 5 days after that, SFO/BOS still hasn't cleared (last time the GS desk sent an itinerary to IM for me, IM cleared all my upgrades on that itinerary within a couple of days of receiving the itinerary).

So, somehow, with a higher MP status (GS vs. general), a higher fare (B vs. Q), and applying for the upgrade 2 days before my wife, she still beat me to the upgrade on the same flight...

No complaints, since we're better off that way (in theory, I'm more likely to clear than her as the flight date approaches... but then again, maybe they're giving away all the upgrades to other people, and another general member on a Q fare will beat me to the next available upgrade), but it does show an issue with their system, on the back side with their inventory management or upgrade triggers, not on the front side (UA.com).