So let's say Vegas, with their uber capping skillz, decides the Mavs will win this by a point (which they did decide, given the spread). Why would Vegas not put this game at -4 or -5, KNOWING that 80% of the public will still think the line is too generous and pound the Mavs. Then when the Mavs win by 1 as predicted, Vegas makes a ton of money.
My point is, what is Vegas trying to say with a -1 line?