12 vs. 6?

This particular index play has me confused as to why the numbers are the way that they are. For the Hi-Lo count, DD, the index numbers are hit at 0 or less S17, hit at -3 or less H17. Intuitively I would think that these numbers should be reversed since it is possible for the dealer to has a pat hand and that you should be more aggressive, not lees, when facing this rule.