Oddly enough the've ALWAYS done this at Prestwick going back to 1990 at least when NATs were running the show. I know because I was doing my O Grades at the time and got puzzled by the fact that the Runway 31 elevation was 66 ft ( it is now 65 ft I believe - is the country sinking we need to know? ) and the other end at 13 was only 38 feet. There didn't LOOK like a 28 ft disparity but I was only a school boy with an air band radio in those days.