Sometimes you do not discover a mistake until you are finishing a sweater. This button band was knit too tightly, meaning that too many stitches were picked up.
The result is too much material between the buttons, which causes puckering. There is a bubble between the 2nd and 3rd button, the material should lay flat. Attempting to stretch the button band more between buttons to force it to flatten out will result in the buttonhole side of the cardigan being longer that the side the buttons are sewn on to.
The solution, in this case, is to rip out the button band and re-knit it. This time picking up fewer stitches. The overall effect may also reduce the number of buttonholes on the button band. The knitter of this project (Angela) may see the number of buttonholes reduced from 12 to 11, which is what the pattern actually called for and is the number of buttons purchased for the sweater.
Moral of the story: don't pick up stitches too tightly on the button band and read the directions for number of buttonholes the pattern actually calls for.