Office Space
I had always assumed that the points were solely based on your monthly spend. (How they round...I never really cared.) I have three rewards cards - HH Amex, SPG Amex, and MR Visa.
From an IT perspective, it seems expensive to push points into an individual accumulator every time a transaction takes place. Far easier/cheaper to simply grab the monthly total at the time the batch job runs to build the outgoing file to the airline or hotel in question.