Measuring sea level change is difficult even with the best of modern instruments. Figuring out the sea level thousands of years ago is even harder.
We think of the rocks that form the sea bed and continents as hard, solid, stable things. In fact they are not – over geological time scales, the rocks move, sometimes a lot. I got a first-hand look at this on the Baltic islands of Estonia, especially Saremaa and Hiumaa. These islands are rising out of the sea, growing a little higher and bigger each year. If you lived there, you'd probably thing that meant the sea level was declining – that's what it looks like to you, standing on “solid” ground. But in fact, the limestone that makes up those islands is slowing rising – and in this case, geologists know exactly why. During the last Ice Age, an enormous mass of ice on the land pressed it down, displacing magma deep under the crust. After the Ice Age ended (and the ice all melted), that weight was lifted – and the land is slowly resuming its natural level. Geologists estimate that for the next 100,000 to 400,000 years, the land will continue to slowly rise. And residents will probably still think the sea is receding :)