Why does OP.GG show different stats than other sites?

The stats vary from site to site because the amount of data stored in the database differs, and all the services are fetching data only for summoners whose users clicked to update due to Riot Games' server load limit.

So basically, the more users use our service, the more data we can fetch, and we're updating more than 99.8% of the games used for champion analysis, which is the world's most popular League statistics website.