While the assumption is that we should get 3 extra points for each dollar over the specified amount (which varies quite a bit), there are two variables that also apparently come into play:
1) The bonus only starts on the day you registered (which in my case was about two weeks into October), even though notification about the promotion didn't happen until about three weeks after the start of my October billing cycle.
2) The bonus apparently is based on calendar October/November spend, rather than on the total of your October and November billing cycles.
You may want to call AMEX to sort it out.