? for SW or Don S - Hard 12 versus Dealer 6 index S17

Hey AP's,

What is your take on this index #? I understand 12 v 4 being zero. 12 v 5 being -1. But I have a hard time understanding the math behind the hard 12 vs 6 being zero on a S17 chart in Appendix A p. 267 in Wong's Professional BJ.

Any insight to this would be helpful!

Thanks!

MC