Higher Elos and smurfs.

When I think about people in Plat and above, everyone and their neighbor has at least one smurf. And since plat accounts for 10% of the league community in NA. That is at least another 10% of low elo smurfs which will eventually climb the ladder.

So in Silver and gold theres a pretty darn good chance you might come across some smurfs. Since it is 10 people to the game.