Because land rises.
During the ice age the land was depressed - it is still slowly rising now that that ice has gone. It's called post glacial rebound
http://en.wikipedia.org/wiki/Post-glacial_rebound
I don't know what it is in Wales but in some parts of Finland it's 11mm/year
Currently sea levels are rising at about 3mm a year although this could accelerate depending on what happens
Obviously countries like Bangladesh and Pacific Islands cannot rely on PGR to protect them!