The seam falling to the back of the shoulder is to create a bias edge on that shoulder seam. This helps the fabric mould around the shoulder, giving a tighter fit.

La couture tombant à l’arrière de l’épaule vise à créer un bord en biais sur cette couture d’épaule. Cela aide le tissu à se mouler autour de l’épaule, donnant un ajustement plus serré.

Merci
Dominique from Belgium

Yes it can if you don’t pay attention. One thing is to make a note of your bust, waist (and hip) measurements while in your corset for the first fitting. Each time you fit another mockup or wear the final bodice take measures again to insure you are at the “original” size. Here’s my post on ease to help you build that into your final bodice to allow for (very) minor changes.