The same logic where more people are likely to post negative reviews than positive ones. You're probably right, to a degree. I think there are also plenty of people who test here on sunny days. If that is true, it's true across the board with all providers. They're all being judged using the same method. In my experience, the better providers always perform better and rank higher here. Is any of this perfect, no. Will it ever be perfect, no. Will I try, definitely.
After TMN's recent full switch to https (SSL) settles in with the search engines I have an upgrade to the host stats pages you mention. It aggregates a much broader range and I think it better represents the highlighted ISP, city or country's speed because it won't just be showing you the logs like it does today. It will unlock a huge amount of information that TMN has been calculating and storing in private databases for a very long time now. So look for that tool to become higher resolution in the near future.