Different index numbers

Hi all, I noticed that High-Low index numbers are not the same in Wong's "Professional Blackjack" (PB) from 1994 and "Blackjack Secrets" (BS) from 1998, though the game and the definition of the index number is the same (truncating and not rounding).

Example: Multi-deck S17, 12 vs 3
PB tells us 2 (page 267) but BS tells us 1 (page 56).

Is it because BS is newer?

Thanks a lot and sorry if this had already been clarified before.