Is StatCounter reliable ?

From the data of StatCounter.
http://gs.statcounter.com/#search_engine-JP-monthly-200807-200912

StatCounter is free tool and they provide the share of OS, browser, search engine, so on. The following chart is search engine share in Japan by StatCounter.

But as we Japanese know, Japanese top search engine is Yahoo! Japan, not Google. From NetRatings Japan or comScore Japan, share of Yahoo! Japan is over the half at least. And trend of last year is not stable. Is this data statistically reliable ? Umm.