That's easy :
1st one is Windows, 2nd one is Mac OS X, 3rd one is Ubuntu and they are almost as popular as Ubuntu according to distrowatch, so they are 4th.
While that's a joke from me and I am in no way affiliated to linux mint and completely clueless on that point, I strongly suspect that's the real explanation.
Counting Windows as a single OS would be false IMHO, so I suspect that the 3 first place would me microsoft products ( xp, 7, and 2k8 ). That let OS X as 4th, and likely 5th, or vista. Then we can start to find various Linux distribution and based on my own experience at the lug and various events, Linux Mint is no in the top 5.
What amaze me is the number of comment on the blog, and the fact that no one question neither the stats, nor the whole business plan and the lack of detailed financial report ( cause 3500$ a month is enough to pay one person full time, and that's not negligible ).