Instead, they want you to take the -9 spread and land on -3, giving them extra points of cushion.
In short, they don't want to give you a -7 spread that you can use a six-point teaser on to bring to -1. However, you are required to pick from the spreads that are displayed on the default screen, and this is why books throw out non-key numbers with lopsided juice.
You pick a set number of points and a certain number of teams, and each spread is moved in your favor by the selected point amount. For the uninitiated, a teaser is a combo bet (like a parlay) with fixed odds (unlike a parlay).
