That shows the difference between standard time and local apparent solar time. Apparent solar time varies through the year according to the equation of time.
This map shows the difference between standard time and local mean solar time, which is fixed. There is a clock on the Corn Exchange in Bristol with two minute hands with a fixed angle showing the difference between Bristol and Greenwich mean time. The clock dates from the period when Railway Time (GMT) was the practical time in common use, but British legal time was local mean time.
