Loren Coe writes:

Richard seems to be saying the internal
test signal is sync'd on the line frequency, more than a few microseconds
variation would be unexpected.

Correct, this is the point of having it as a trigger reference, to see what
devices are doing relative to the wall outlet.

assuming this is a split phase supply (120/240), theoretically 180deg
phase shift would = 0 watts, right?

I would expect it to be the absolute value of the cosine, which is zero at
90 and 270, and 100 percent at 0 and 180.