9-slice not scaling width properly

I’m making a slider component, and I have a vector Sprite highlight on it that I’m scaling with 9-slice. It works pretty well, but the farther it stretches, the more imprecise the stretch gets.

In other words, it doesn’t stretch the graphic to the exact width specified. It’s like, 95%. And as the width gets bigger, that empty 5% gets more and more noticeable.

It seems like Flash is including some empty space, thinking that it’s part of the graphic. There’s only one layer on the highlight art, and I’ve verified that changing the art doesn’t solve the problem.

I’ve also verified that the width that I’m setting the highlight to is the correct width. But the art still isn’t wide enough.

Any ideas? I’m stumped.