They are a composite of every card that can make up that play. There are a number of ways to make a hand of 16 v 10 up, and there can be any number of cards played prior to getting that 16, so the computer will calculate every possibility and spit out a number. They are basically accurate, but rounding them up or down will not make that much of a difference. Some players like indices that are -6 -4 -2 0 +2 +4 +6 to make it easier for them to remember them.