Ok, I'll give you the benefit of the doubt there. The odds of it being dummy bookings is extremely low. I'm a software developer, and we have 3 test systems I can think of off the top of my head. Anyone who would need to make dummy bookings would probably do it under a test system, or if they must test in production, they'll use a flight far off and cancel it ASAP. As you can tell by our reactions, inventory is a precious thing to keep on top of. So the odds of a dummy booking opening seats at the last minute is very low, especially for the flight you happen to be on. But a misconnect, especially when flying from a hub (such as LAX)? Certainly possible. Or a business traveler whose meeting went late. Or someone stuck in traffic. You get the idea. Point is, it's certainly possible that seats will open up last-minute, but not something to be counted on.

Right now the UA flights have 4 seats, 2 nonrevs, followed by 10 seats, no nonrevs. Which is roughly what I saw a couple days ago and eventually edited out.