The only thing I can imagine is that it measures sum(max(clientSpeed,NetflixSpeed))/n , where NetflixSpeed is only about 4Mbps. That would make this a ranking of what isp has the lowest amount of users with really shitty WiFi, or users watching netflix on slow smartphones. I doubt there are many ISP's with offer a connection of less than 4Mbps between their backbone and Netflix.
Either way, it's good to see my ISP ranking the second highest worldwide.
If you click on any details page (either the global chart or country full results) and scroll to the page bottom you get an explanatory text:
> These ratings reflect the average performance of all Netflix streams on each ISPs network from Nov. 2012 through Sept. 2013 and average performance during prime time starting in Oct. 2013. The average is well below the peak performance due to many factors including the variety of encodes we use to deliver the TV shows and movies we carry as well as home Wi-Fi and the variety of devices our members use. Those factors cancel out when comparing across ISPs, so these relative rankings are a good indicator of the consistent performance typically experienced across all users on an ISP network.