It's likely 76% that have google fiber, and a router that supports it and their computer setup to work with it. Since it's measured by sites actually reporting

It's likely 76% that have google fiber, and a router that supports it and their computer setup to work with it. Since it's measured by sites actually reporting the traffic on ipv6 vs ipv4 the ones not on ipv6 are likely those with setups that are preventing proper support.