Alarmists cheat with almost every measurement, but none more blatant than the global isostatic adjustment (GIA) scam.
The theory is that due to post-glacial rebound, land is rising and the sea floor is sinking – making the oceans deeper. So they add 0.3 mm/year on to compensate. This might be a valid consideration when calculating ocean depth or volume, but it is 100% fraudulent for calculating sea level – which is the essentially the distance of the sea surface from the center of the Earth.
The only theoretical reason to do adjustments, is to compensate for measurement error. But there is no claim of measurement error associated with GIA. It is simply an excuse to further jack up the phony sea level data required by politicians.