From the previous 10x offer, my understanding is Amex does 3 calculations:
1) Purchase price @ 1x -> Rounding (1x points == round purchase price to nearest dollar) -> Posts as normal
2) a) Purcase price @10x (or 15x) -> Rounding (10x means multiply purchase price by 10, then round to nearest dollar)
b) Subtract posted points calculated @ 1x
c) Post the remainder from the subtraction as bonus points

This works exactly for the days when I only have one bonused purchase. I haven't done a full reconciliation, but I understand that it consolidate bonus points from multiple bonus'ed transactions on the same day