Any tracking service has to use "guessing" based on past trends.
Take NPD. They alledgedly use 60% of the retailers' data. I assume the other 40% of their figures must be not only extrapolated by a mathematical formula, but also subjected to adjustments based on the demographics a game attracts. For example, you'd expect a title like Ninja Gaiden 2 to sell much better at Gamestop than Walmart. On the other hand, a title like Wii Play may sell better at Walmart. A simple mathematical formula can't embody all of these concepts unless it includes some manual parameters which are input by subjective humans.
Of course, I myself am guessing NPD's methodology which AFAIK is not public either. There could be a lot of guessing there too, and we'd probably never know. Of one thing I'm fairly sure though - NPD doesn't have a magical accurate formula to get that 40% of the market based solely on the other 60%.
My Mario Kart Wii friend code: 2707-1866-0957
