I'm relatively new in AP and still trying to grasp all the details. For example, in Basic Strategy, whether you should DD with a 9 vs. a Dealer 2 depends on table rules. If you are playing a double deck game, DHS17, DAS allowed, you SHOULD DD on a 9 vs. Dearer 2. In looking at the Illustrious 18 chart, Player 9 vs. a Dealer 2 is a +1, regardless of # of decks. So in this case, is the I18 saying ONLY DD if the TC is +1 or higher? If so, why does BS say DD to begin with?
BS assumes that you are being dealt from a newly shuffled deck. If you hold 9v2, your hand is going to be 2 7, 3 6, or 4 5. Including the dealer's 2, your running count will be 2 or 3, so your TC is +1 or +1.5.
All this aside, if there is a discrepancy between BS and index numbers, use BS for the first hand only (assuming you can't see anyone else's hands) and index numbers otherwise. This is because index numbers are sometimes a compromise, whereas BS is accurate for the first hand.