If Rentrak includes bundled software, that would explain a few of the differences. I'm not trying to suggest that Rentrak should be considered reliable, just that the little data we get from each company makes it pretty much impossible to compare the two. Each needs to be considered on it's own merits.
(Plus I'm holding out hope that the weekly top 10 actually means something
I'm not trying to say Rentrak is less reliable than NPD at all. In fact, if i had to be, I'd bet on the opposite. =P
I agree it's not possible to make any good comparison. I'm just pointing out the discrepancies.
Edit: By the way, latest Rentrak weekly chart:
Video Game Sales Week of Jan 29-Feb 4
1 PS3 Final Fantasy XIII-2
2 X360 Final Fantasy XIII-2
3 X360 SoulCalibur V
4 PS3 SoulCalibur V
5 WII Just Dance 3
6 X360 Call of Duty: Modern Warfare 3
7 WII Zumba Fitness
8 3DS Mario Kart 7
9 X360 The Elder Scrolls V: Skyrim
10 X360 NBA 2K12