The change from Jan to Feb 2014 alone is impressive:
Comcast: 20.61% -> 24.42%
AT&T: 13.53% -> 16.77%
Verizon Wireless: 40.03% -> 45.75%
Time Warner Cable: 3.88% -> 5.37%
T-Mobile USA: 9.58% -> 15.76%
Google Fiber: 71.87% -> 76.43%
Comcast: 20.61% -> 24.42%
AT&T: 13.53% -> 16.77%
Verizon Wireless: 40.03% -> 45.75%
Time Warner Cable: 3.88% -> 5.37%
T-Mobile USA: 9.58% -> 15.76%
Google Fiber: 71.87% -> 76.43%
Network providers sign up to join the measurement effort and then Google, Facebook, Akamai and Yahoo measure the connections coming from those networks over IPv6 or IPv4. A person at the Internet Society then aggregates the data from the providers and makes it available on the World IPv6 Launch measurements page. The specific mechanisms used to measure IPv6 by the 4 companies are described at the bottom of that measurements web page.
My guess would be that while all Google Fiber customers very well may have IPv6, they may be using some devices that are only IPv4 capable.