Not too keen on AM/PM session grading but the best of a bad solution.
Comps should be the same for all which you will never ever get that with two or more sessions. Likewise I would hate to go round in threes or fours in one session.
Its a rarity for pm to have the better conditions, its a known fact wind gradually increases throughout the day making the same course harder, hence am always fully booked first.
What it really does is that persons grading in nearing a lower grade score.
Prime example today Simon well done those that scored over 30 in am being quite windy, pm substantially windier plus gusts, rain and ground the bag slid on not to mention the extra wind chill.