Its the weather 'changing' that is causing this and I presume it is no coincidence the best lap times are all set with the same 'weather'.
Download the top replay, and enter the event. Try to load the ghost and if it isnt found the weather is wrong. Exit, reload and try again. Eventually (It took me ten goes on one occasion) your weather will be the same as the leaders and you can then compare your data to the leaders.