Actually, yours would make more sense to me, especially if you are getting a different variance each month. If they are rounding $1.62 to 2 points, and $1.41 to 1, it would come out close to even each month, but a little off. The more purchases, the less likely you will come out even, and the better chance for a big difference. I just got my AmEx a couple of months ago, so I don't have much to check at this point.