My original guess was that it was the number of substrings that are common 3-or-more-letter English words (with some sort of objective rule for what counts as "common"):
GRAVE/RAVE, HER/AND, ???, KIN/KINS, BEN?/NET, SON/???, SHEER/RAN, TRAIN/RAIN
but that breaks in a number of places, so it's pretty clearly wrong