Going to go with a couple of wild ass guesses here:
There is likely some elasticity in the online reservation system, if someone has reserved a mini, they haven't shown up to pick it up, that frees up stock Apple is holding, or they know they're getting a shipment the next day and reserve one of those units for the person that has a reserved unit, and I suspect that when they pull the minis from the back to put out front on the express table, they're taken out of the "available" inventory, and assumed sold.
I live in Los Angeles, and haven't been able to reserve one for local pick up (LTE any capacity) since they've been available, finally got one from Mac Mall. (or at least I hope I did, it hasn't shipped yet).