Casting off confusing

Shaping neckline: Pattern reads cast off center stitches as follows: K2tog slip st on RH needle to LH needle. The way I read it is to knit 2 together then slip the stitch to Left hand needle. But that is not casting off center stitches for neckline. I will still have stitches left in center. I attached directions and diagram. Can anyone tell me what this means.

If you keep repeating that until you’ve bound off 23 sts (for your size), you’ll decrease the center sts. It’s a very nice bind off.
If you don’t like the look of that bind off, you can always use a conventional bind off (lift one stitch over the last one knit).