From the docs:
Where T is the total number of elements in both sequences, and M is the number of matches, this is 2.0*M / T.
In this case, T is 17 because the first string has 8 characters and the second string has 9. M is 8 because eight characters from the first string match with characters from the second. 2 * 8 / 17 equals 0.9411764705882353.