That 5% number is meaningless.

Lets start with what percent of all people who own computers have a Nvidia graphics card. Considering that the mass bulk of computers have a crappy integrated chipset, and there is also ATI/AMD out there, I can safely say <50%.

Among those 50% what percent are Linux users? Wouldn't it be expected the kind of people enthused enough to buy a discrete graphics card are also the kind of people with the interest to try Linux?

I imagine it is a number higher than 5%, maybe only 10 or 15%, but it is higher.