Why A7 vs A at TC>1 become stand in S17

[PromVRT]

I see from Stanford Wong at in S17 game for A7 (soft 18) vs A, at TC above +1 the strategy becomes stand instead of hit. I checked with Wizard in the calculator by reducing the number of low cards outstanding, the strategy really changes from hit to stand.

Basically, I do not understand the logic. Soft 18 vs 9, Ten, the reason why hitting is optimal is the likelihood of getting small cards like 2 or 3 while getting the picture neither hurts or help. The ace slightly helps. But with the ace, hitting soft 18 in high count is more likely to catch the picture or the ace to soft 19, so it is likely hitting does not really hurt. The 6? It helps as more 6s cause the dealer to stand on 17 more often but on the other hand the player does not like the 6 after a hit as well to hard 14. Moreover, high count also suggests 6s are depleting too. If the dealer gets a BJ, both result in a loss anyway. Why the strategy becomes stand then?

Many thanks