Yes, the back armhole shaping at 6.75" will decrease the total number of sts to 15. You need to shape the armhole at each front. But the pattern still doesn't make sense and no wonder you were confused. The cast on is 33 for the front minus 5 for the armhole and then 13 for the neck shaping and you get 15sts, not 11.
You could write to Universal Yarn
or you could try to work in some extra decreases in the neck, say bind off 3sts twice instead of 2sts and then extend the rows with 1 decrease for 2 more rows. Ending with 11sts would match the back shoulder.