Why are the stats from OP.GG different from those on other websites?

The stats from OP.GG and other services are based on the amount of data stored in the services’ respective databases. To lessen the load on Riot Games’ server load, each service can only pull data from summoners who refreshed their status on the service. 

This means the more users a service has, the more data that service is able to provide. OP.GG currently provides 99.8% of the games used for champion analyses.