This pattern comes with a default 0.5 cm scye dart (think armhole dart) on the back. You can increase or decrease the dart to influence the armhole.
The point of this dart is to make sure the armhole lies flat against your body, rather than gape. Given that there is no sleeve, a gaping armhole is particularly unsightly.
While this dart is used to shape the pattern, you don't have to sew the dart because it's incorporated in the shoulder seam line.