I struggled with this as well!
You have to save MULTIPLE "GHOST" replays in order to have reference data.
Ghost Replays save all of the race data, actual race replays... it's just a video.
Reading the manual it states you can only load reference data for the same track with the same weather. Because the weather for this event is set to variable I am thinking it prevents the loading of reference data???
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.
Are you suggesting that there is some weather conditions more favorable to set a good time and this is ramdom?
Could it be related with ghost replays freezing the game? You can download any replay in the top times list, but maybe only some of them meet the weather conditions you are playing with. This could make the game crash...