I don’t know about United’s specific figures but assume:
DOW 130T (Typical config)
MZFW 181T
Fuel in tanks 85T (max is about 100T but 85 should be enough for this trip)
MTOW 247T (possibly 250T, depending on options)
leaves approx 32T for payload and nowhere near MZFW.
Typical pax config is 220-250, which would weigh about 22-25T, so should leave 7-10T for freight.
I was under the impression that Los Angeles was well within reach at max weight and that MZFW would not have to be reduced if carrying full fuel.
Yes, LAX is well within reach with less than full fuel (even if at MTOW) but you could not do this at MZFW. If for some reason you did load the plane up to MZFW (approx 181T), then you could only take 66-69T of fuel, depending on your airline’s MTOW option. This is still enough to fly for about 12 hours, plus reserves.
If they did load “too much fuel”, then they may have had to reduce the payload.
Let’s say they loaded full fuel, 100T. Added to the DOW, that would give a weight of 230T which would only leave room for 17-20T of payload.
Defueling could be an option but this might be difficult to arrange, as some fuel suppliers are reluctant to do it.