NPD does market research for a ton of markets, not just videogames. If they got caught fudging on numbers in one area, they would lose customers from other areas where there may be competition. They also have to compete with websites like this one and with each companies own internal tracking, meaning they have to assure that their numbers are the MOST accurate--enough to be worth hundreds of thousands (and maybe millions) of dollars. What's more, if they were known to be faking numbers, the ESA would likely establish an alternate system, akin to RIAA Gold and Platinum certification. Lastly, companies would already have to be cooking the books in the first place to need this. They stand to gain very little from a marketing standpoint, and it would be obvious there was error if they weren't already misrepresenting their sales to shareholders.
"[Our former customers] are unable to find software which they WANT to play."
"The way to solve this problem lies in how to communicate what kind of games [they CAN play]."
Satoru Iwata, Nintendo President. Only slightly paraphrased.
