Vgchartz vs NPD numbers for February 2009

Vgchartz has posted an article on the difference between their numbers and NPD

..........VGC ........... NPD
wii..... 752,972... 753,000
Ds..... 497,174 ... 588,000... (approx. 91,000 difference)
X360... 315,992... 391,000 ... (approx. 75,000 difference)
PS3.... 204,218 ... 276,000... (approx. 72,000 difference)
PSP... 189,106 ... 199,000... (approx , 10,000 difference)

Street Fighter 4
360 vg 295k / npd 446K... (approx. 151,000 difference)
PS3 vg 182k / npd 403K... (approx. 221,000 difference)