From the FAQ:
Sites with Custom Fonts
This chart shows the percentage of sites that make at least one request for a custom font. The determination of a custom font request is based on the Content-Type response header. However, since many fonts today do not have the proper Content-Type value, we also include requests that end in “.eot”, “.ttf”, “.woff”, or “.otf”, or contain “.eot?”, “.ttf?”, “.woff?”, or “.otf?” (i.e., a querystring).
If the criterion is “ends in” then .woff will not match .woff2 which implies that the numbers for downloadable fonts are too low. This is particularly problematic given that Google fonts serves WOFF2 to Chrome (in preference to WOFF) for the past three years.