Nope. Our creeping delay the other day was due to the incoming plane from SAD being late. For us it wasn't much more than a 30 minute delay and went exactly as rch4u describes above. Having already flown LHR-ORD we did notice the extra standing around more than we might have otherwise. We've also experienced the fuller range of incremental creeps as described by the OP.

I still have the impression UA could provide a more realistic departure time when the delay is due simply to a late arrival that presumably has a landing slot and a reasonably accurate ETA, but then again I'm not in the airline business.

We also experienced the opposite a few years ago at ORD. The GA announced we had at least 45 minutes before we could expect to board and sent the waiting pax on their way to restrooms and fast food with her blessing. We happened to hang around and less not much more than 5 minutes later she suddenly starting boarding. IIRC correctly it was a light load. Ever since then I've been inclined to hang around the gate like a dirty shirt.