The same thing we do every month, Pinky...
This test is kind of ridiculous. It measures the average streaming bandwidth your customers achieve from Netflix, and Netflix has a maximum streaming rate that simply isn't all that fast. The primary factor controlling an ISP's average score is the average rate a subscriber chose when signing up with that ISP. An ISP can have perfectly good reasonably-priced 50 MBit/sec plans, which would easily stream at Netflix's maximum rate, but if they have a $1/month 3 MBit/sec plan and everyone switched to it, they'd lose the competition.
In other words, there is no particular reason Google Fiber is winning at this one and we can expect their score to decline as they move into neighbourhoods where a greater fraction of people choose the free plan.