The only thing i can say is that the tracking for this game was bad.
220m in sales day 1 equals to 3.3m and the >300m on week 1 points to > 4m in sales. i thought VGC would know this by now and also keep in mind that over 4m players played this game in the first five days, still many milions not conected to Xbox Live.
I will wait for official data since i dont believe this game sold less than Halo 3, meanwhile i will be playing this masterpiece.
I think 4 million players doesn´t neccessarily equal 4 million different players ;)