The in seat DC supplies are limited to 75W, I don't know about the ones that have 110Vac at the seat, but I would imagine the limit is similar if not the same.
The PSU for the HP is probably overrated, it will have enough to run the laptop (say 70W), but to charge the battery whilst running (say another 20W) plus enough to power attached devices (USB x4 at 5W each). So 120W isn't too surprising. The laptop probably averages much less than that in normal usage, probably down nearer 30 to 40W, but the problem with laptop PSUs, especially nice chunky 120W ones, is the "Inrush" or "Surge" current. This is the very high current that occurs when the PSU is plugged in. It only lasts a few milliseconds, but is often enough to trip circuit breakers, which is why "Slo-Blow" fuses exist.
Sometimes this can be overcome by plugging in, and reseting the breaker quickly as soon as it trips. I.e. Using the short time before the breaker trips to supply the inrush current. Whether or not you could get cabin staff to do such a thing is another question.