Would the track day organisers re-time people throughout the day? and move you around accordingly.
If for example I am doing a track day at a circuit I know well I will be going fairly quickly in the first session just to bed myself in and build up pace quickly as each session passes. Whereas if I was at an unfamiliar circuit I would be more tentative and build my speed up slowly and progressively throughout the day and probably by mid-afternoon I would probably be within 5-10% of as quick as I could possibly go on subsequent days there.
So this would raise the issue of say me at a track that I haven't been to being timed initially and hopefully by the end of the day being anywhere from 10-15 seconds quicker, others who may have done the circuit before might not progress at all, some may go slower as their stamina goes etc, etc, quite a lot of factors just to group people closer together.
As Henry has said also, some people can only thrash it down the straights, and slow through the corners, and vice versa, so even though the times maybe similar, the actual running order on the track could become infuriating for the riders as they swap and change places over and over which to some is entertaining to others could mean pushing that bit harder and ending up in the kitty liter