questions about reko, ko indices

In the ko book, it says there are indices for single and double deck for 12v4, 12v5, and 12v6. In reko, there are indices for this also. I don't understand. I thought that basic strategy tells you to stand on these hands anyway. It couldn't possibly mean to hit at counts at or above the index.

Also, in ko full, the indices are different than the preferred. I thought it just added more than the preferred for example 16v10 at -8 in full and -4 in preferred for 6 deck.