The problem is that oceans fluctuate in level. There are tides, both Lunar and Solar, wind based rises, and many other influences. What is the real high level? What is the real low level? is it just where the highest storm wave reached? Is it an average? What about years with higher velocity peak winds?
I have never seen any measurement that gives the uncertainty level for estimates of sea level. That is a cause for concern. There is uncertainty in ALL measurements.
For this, I suspect that the changes are smaller than the uncertainty. That means that they really don't know. for the South Pacific measurements you mentioned, those were done by satellite, and so reflect many different things, such as tides and wave heights at the time.