Elo is dependent upon the player pool. Claude Bloodegood [1] took this to an extreme, nearly becoming the highest rated player in the world at the time, largely by playing officially rated games in prison against other players who were completely inexperienced but whose rating did not reflect that since they were, in turn, also only playing against completely inexperienced players.

Online ratings have a correlation with otb ratings, but they're very different - especially on the lower (and extremely high) ends.

[1] - https://en.wikipedia.org/wiki/Claude_Bloodgood