My guess is that Netflix is reporting the average stream bandwidth accessed by customers of each ISP. So if video X has streams at 500kbps, 1Mbps, 2Mbps, and 5Mbps, then a higher percentage of Google Fiber users being able to reliably stream the 5Mbps feed instead of just one of the lower-bitrate ones would result in a higher average.

So there would be a built-in ceiling of the highest bitrate Netflix offers. And based on this data, I'm guessing that for a bunch of their content, their streams max out around 2-3Mbps (which is a reasonable bitrate for 720p film/TV content).