Okay so this could be wrong, but I was speaking to my friend a Duke undergraduate and he gave me his theory about this insanely high reported IS number. He told me that almost all Duke undergrads who apply to Duke are given an II, and that this large cohort (close to 200, he said) might be incorrectly reported to USNWR as all in-state applicants.
Again, this was just speculation on his part. Also I don't want to seem like I'm discrediting Duke undergrads who interview here - it's amazing university and I'm sure you guys are stellar applicants, too.