[arethuza]

Relative sea levels can change because the land is rising as well as sea levels dropping - post glacial rebound means that areas that were heavily glaciated can actually be lifting at quite a rate:

https://en.wikipedia.org/wiki/Post-glacial_rebound

e.g. Here in Scotland you just have to look at a shoreline to see that relative sea levels have dropped quite a bit even though sea levels have risen. When I was a kid I can remember my father talking about legends of Vikings sailing round Roseisle in Moray - which is quite high and dry now.