What is wrong with Xbox360 sales in NAmerica

[libellule]

 

hi all,

I thought gap between PS3 - Xbox360 consoles was really important  as you can see here :

 from VGC, addition of weekly sales of May :

Xbox360 : 241 000

PS3 : 84 000 (3 fold difference)

But If I refer to this (good/no-biased) french site (stats from NPD i think) :

 Xbox360 : 156 000

PS3 : 81 000 (2 fold)

http://www.gamekult.com/articles/A0000058546/ 

And I saw in the commentary that I was not the only one to wonder about sales comparaison between NPD and VGC.

What is the problem about Xbox360 sales ?

I thought the only problem was PAL sales but it seems that there are some problems to report sales from NA ? 

Just a question to better understand these type of sales report.