I'd say your math is right as I understand patterns. If Isabeau is right, Moda Dea is using bind off differently from all other pattern writers in the history of the universe. But they're consistent across the sizes, so it's not just a mistake.
Have you tried writing to them? I don't know how they respond to customer questions, but they have a "contact us" link on the pattern page.
Since the directions for the back leave 9 stitches left at the shoulder, I'd consider doing two fewer decreases along the way on the front to match the back. You can probably do one less neck decrease and one less shoulder decrease for it to work.