It's updated each month. I presume it records the hardware profile of folks who use it.
You do have to opt in, and I don't, but many people do.
As stated, AMD has infact got the larger market share, and that happened in a quarter in which the 480/470/465 GTX had been out and the 460gtx's were out during the majority of it, the 430gt was I believe on sale for part of it to OEM's only.
THere really aren't ANY companies nvidia has locked in any more, at all, because most companies absolutely do their best to stay away from them.
Dell, nope, Apple, nope, Acer, nope, etc, etc, while 3-4 years ago these guys were 100% Nvidia(or as good as) they simply aren't anymore, they aren't even remotely close. I'm not sure theres a single top tier exclusive Nvidia company at all anymore, so no their brand doesn't just sell.
Nvidia branding does work, on fanboys who upgrade their own cards, it works on AMD aswell, us guys make up a number of sales that could pretty much be a statistical error. Most sales go into prebuilt computers, laptops, consoles, Nvidia has no strangle hold anywhere and is heavily heavily rumoured to have lost pretty much every single console for the next gen.
Though that has slightly changed, the consoles were pushed two years back, hardware will change but its FAR more likely that Sony will go with AMD over Nvidia, its well known Sony aren't happy with dealing with Nvidia, MS made it clear they were unhappy with Nvidia and moved away from them.
Nvidia's "brand" name means nothing with Dell, HP, Sony and everyone else, most of them filed lawsuits and have had multiple lawsuits filed against them over "bumpgate", lost millions upon millions and only gotten fairly small payouts from Nvidia to recoup some of the losses. Their brandname has been dirt with these guys for 2 years, thats why most of them aren't exclusive(or as good as) anymore. Its insane to suggest all the companies screwed by Nvidia are happy and regard them as better, that hasn't been true for a LONG time now.
As for sales, yes you expect in ANY product cycle for ANY product for sales to slow down as you approach a new product, honestly I think Nvidia haven't had a huge amount of influence there, some for sure but not that much.
6 series sales are likely in the first 3-4 months to dwarf Nvidia sales again simple because in the 6 series lifespan from say now till next October, I'd expect the first 3-4 months sales for each segment to be noticeably higher than the last 3-4 months sales.
This is before of course, Ontario/Zacate launch which are almost certainly going to be incredibly strong in their individual mobile fields, Llano on desktop which will eat away Nvidia sales aswell.
The 430gt(which should end up being Nvidia's largest selling product), has worse drivers, worse image quality(for a card aimed at HTPC) uses more power and is significantly slower in gaming while costing more than a 5670. The 5570 matches and often beats it in gaming, costs a decent amount less, and still has superior image quality.
Of course Nvidia hasn't really announced it but they still have the low end 310/320gt's in production as their really low end, I wouldn't be surprised if Nvidia is pushing them harder and if they easily outsell the 430gt. Problem is Nvidia is pushing slower, older dx10.1 2 year old tech vs what in a couple months will be second generation dx11, faster and better low end cards.