The answer for this comes down to three essential things:
- Userbase
- Christmas
- Demographics
The Wii has a larger userbase than the Xbox 360, therefore all else equal you would expect higher sales on the Wii.
The second and third reasons are connected. One would expect that the Wii's audience would be on average younger than that of its competition (due to Nintendo's image, lack of violent games etc) and Christmas presents tend to be directed more towards this demographic than the perhaps older demographics that frequent the Xbox 360. Therefore all else equal you would expect the Wii to get a bigger software boost at Christmas than the Xbox 360.
Couple the effects together and you get the Wii outselling the 360 significantly over the Christmas period.
