It's well understood math, but it's "only" relativistic orbital mechanics.

It boils down to a pretty consistent number, but how you get there is related to the weight of the moon, how far it is from earth, and how fast it's going.
Since the moon is going different speeds at different places in it's orbit, the number actually changes slightly over the month.

They're just using the average though, since it makes life far easier. We use the average for earth too, since clocks move differently at different altitudes or distances from the equator.