For example, outside defense is an important skill for small forwards, but inside defense may have been chosen as one of the 3 SF options instead because more SFs have one of {JS, ID, R} as one of their top skills than have one of {JS, OD, R}. I am just speculating, but this may be because there is a stonger positive correlation beween JS and OD than there is with JS and ID.
Aside from the positive corellation of JS and OD, the more important reason is that players with JS and OD (but no ID) are, I would think, way more likely to be classified as SGs rather than SFs.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."