Gebx -
The December numbers are based on a very simple methodology of PW * 1.2 (prev week by 1.2) up till the 28th, then down 40% the next week. I manually input the Jan-Mar numbers.
For Jan-Mar, I gave the PS3 higher numbers than the X360 had in the same timeframe, which may be high.
So in the case of Jan-Mar, there may be discrepancy, but there's very little chance the X360 won't have a 8.5m lead come January 4th.
Back from the dead, I'm afraid.