Interesting website for me,I worked at a few nuclear power stations when they were being buit,and was involved with moving the first reactor for Rolls Royce Nuclear in about 1964 or thereabouts when it went by train to go into the submarine.Also I have worked on top of a live reactor at Torness Power Station as it was being tested before it went on line.
If you look at this picture of the frequency meter it is slightly over 50hz.When the grid is loaded up as in mealtimes etc.it is slower,so as long as it averages 50hz.over 24hrs.it keeps your electric clocks accurate.
Thanks for the link it takes me back a few years to when I had to make sure our generators were set up correctly initially so they could syncronize with the grid as required.