I'm not for sure about how wordpress would get their stats, but I imagine it's something like scan the internet and put "/wp-admin" behind the base url. If it comes back with the wordpress login page, then count it. At least that's how it seems like hackers find out if you have wordpress from my http server logs.
Also someone posted this thing a while back that does some detection like this to find out what tech is powering the website you are on:
I'm not related to them in any way and I don't know how accurate it is but it was interesting.