A,7 Index/BS Confusion

[SleightOfHand]

I posted a similar post a few months ago when I was still learning CC but never got a straightforward answer. I noticed that the indexes for my system (Zen) are almost always 0 or negative when in agreement with BS and positive when you deviate from BS. However, someone (I don't remember who) commented that this is a common misperception but never claimed why. Looking at some indexes shows that he is right. Ex: My index for Zen on A,7 v 2 is +1 for a 6D H17 game but the BS for this play is to double. Can anyone answer why this happens?

 

[EasyRhino]

If you play a game, fresh off the shuffle, and get an ace, a seven, and a deuce, then the running count is +1.

16v10 works similarly (albeit the count would be -1)

 

[SleightOfHand]

If you play a game, fresh off the shuffle, and get an ace, a seven, and a deuce, then the running count is +1.

16v10 works similarly (albeit the count would be -1)

But the Index is TC +1 (Actually, I think that may have been for a 8D game). Is the decision that close?