ningtong88 said:
If it's based on average, wouldn't it be count 0 then?
Counts do not weigh decisions in this way.
Sure, the Mentor count is balanced, but that only means that it sees a full deck, or six decks, as having a total count of zero.
The actual index for a play is based on all counts and all deck compositions which would occur over at least several hundred million rounds, with any number of cards remaining. The correlation of the count with the play - say 12 versus 4 - can differ, because all the particular combinations of remaining cards which give a particular count will correlate differently with that play than would basic strategy.
Put another way, the tag values give you a different sort of information than does basic strategy. The count oscillates around zero (special case of the True Count Theorem), but this does not mean its values for playing purposes will give you basic strategy, because playing decisions are different from your current advantage, which is what the count most closely tracks.