LC Larry said:
Probably because index play is just an estimate and perfect play take into account every single card that been used
Basically, yes.
Card counting, whatever system(s) of tags and indices you use, has estimation at its heart.
The HiLo tags are designed to give an estimate of the pre-deal EV. And that estimate itself has the potential to be far enough out to cause you to size your bet wrongly in some situations. Using those same
betting tags as the basis for
strategy decisions adds another level of distortion.
Ultimately, I guess the point is that deviating from basic strategy in accordance with an index play will not always yield the correct decision; it's just that it will do so more often than sticking to basic strategy will.
In the two specific examples that were posted -
Doubling 10 v 10 with a TC of +3 :- I guess the main unknown factor will be the ratio of aces to tens hidden in that +3 excess of tens and aces per deck.
Standing on 16 v 10 with TC at -2 :- Similarly, if a greater than average proportion of aces, rather than tens, has been removed to give the -2 count, then that favours standing. There might also be an excess of 6s, 7s, 8s, or 9s that the count tells us nothing about.