To calculate the retention index for a phylogenetic tree, we use the following formula:

$$\frac{\text{maximum number of steps on tree - number of steps on the tree}}
{\text{maximum number of steps on tree - minimum number of steps in the data}}$$

To calculate the maximum number of steps, we look at the number of observed character states for a character and take the lowest number. For each character, we take the lowest number of observed traits and then sum across all characters.

Take the following dataset:

              123456
    Outgroup  000000
    Taxa A    100001
    Taxa B    110001
    Taxa C    111001
    Taxa D    111101
    Taxa E    111111
    Taxa F    111110

The maximum number of steps for the first character is 1, since the character state 0 is attested once and character state 1 occurs six times. 

What I don't understand is why we do not use 6 for the maximum number of changes for the first character.