Well, ratings are just relative to the players (and smurf-accounts) that are present on the platform. Steam has a lot of new players, so of course a 18xx in a pool of new players is less skilled than a 18xx on a platform that's been around for ages.
That's true, but not the point I wanted to make.
The user before me said he is 17+/18+ on Steam and could beat a 25++. This could never happen on Voobly. That's what I meant, regardless the fact that a 17+ and a 25+ on Steam are obviously lower skilled that the equivalent rates on Voobly.