Most people who bet a lot know the Steelers rarely cover. They are a very popular team and collect a lot of 'dumb money' from tourists who bet with their heart and lay money down on the Steelers simply because they are their favorite team. (This also happens with Cowboys.)
Vegas will then adjust the point spread with Pittsburgh as necessary so bettors put money on their opponent with the goal being an even amount of money on both sides ensuring they make as much as possible.
For years the worst kept secrets in football wagering was that the Steelers never cover and ghe patsies always cover at home but never cover on the road. Spygate exposed the patsy 'secret' but the steelers not covering the spread generally holds true today.
They should have won by much more than 3 for sure but the fact that the Browns beat the spread is nothing to party about. It happens more often than not.