Lines don't stay still. From the moment a sportsbook posts a line to game time, that number can move — sometimes by a point or more.
Most common reason: lopsided betting. If 80% of bets come in on the Chiefs -3, the book moves to -4 or -5 to rebalance risk.
Second reason: sharp money. When professionals bet large on one side, books move the line out of respect — because sharp action tends to be right.
If 80% of bets are on the Bills but the line moves toward the Chiefs, that's reverse line movement — a sign that sharp money is on the Chiefs despite the public being on the other side.
Public money moves lines gradually. Sharp money moves them fast. Reverse line movement — where the line goes against the majority of bets — is the clearest signal of sharp action.