I'll leave it to Ryan to explain that number, but were I to guess I'd say it was based on the extensive survey data WotC collected circa 1998 or so, in preparation for the new edition.
For interested folks, I believe Erik is referring to this:
Adventure Game Industry Market Research Summary (RPGs) V1.0 -- Wizards of the Coast
I noticed that WotC is now quoting 6 million, and have no clue where the additional 1 million gamers came from (except perhaps an extrapolation based on the time that has passed since the original survey).
Looking at the original results, the estimate was 5.5 million people played or had played tabletop RPGs in general. If one is going to report only one significant digit, the common thing would be to round up to 6.
Put simply, I don't have a tremendous amount of faith in that number, since it's obvious that no one surveyed everyone in the world and it's thus an extrapolation, or "best guess" based on other data.
Dude, the validity of taking a small sample to represent the population as a whole is fairly well established. If you are going to critique it, you can question how they chose their sample, reported, analyzed, and so on. But failure to survey the six billion people extant is not a strong knock against them.