When you bind off the initial stitches, you end up with two separate halves, each worked with it's own strand.
When you bind off at the neck edge, you only bind off when you're starting the row at the neck edge.
So on one side of the neck you'll only bind off with the right side facing you, and on the other side of the neck you'll bind off with the wrong side facing you.
You can't bind off at the end of a row because you end up with that one stitch you're holding on your needle.
Am I being clear enough?