That's the big mystery. Some of it is in the difference between ECN trades and actual MM trades. ECN trades can come in and post in AH even though their actual trade time is before the closing trade. In this the MM has control of determining the final closing trade. I'm not sure I believe the table of "Clues" for trade amounts. I think it is just that the minimum "all or nothing" trade is 300 shares so the 400 share trade is the smallest that can be effectively used to stabilze price.