I think it is likely a combination of factors.Why do you think that is? Honestly. Why would a kid's ranking drop after committing to UNC? Think about it.
And I tend to agree with you that it does.
It seems to be a relatively recent thing corresponding to the growing deification of the OAD/shoe culture, i.e., how soon will Player-X be able to cash in?
That explains why a Jaylen Brown (whom I coulda told you would be only a moderately effective college player as a freshman) was ranked so high --- he fit the NBA prototype and thus was a sure OAD. It would also explain why there may be a perceived uk bump.
Another factor as someone alluded to may be the current makeup of "deciders" --- I can't attest to any numbers but the impression that I get is that there are more out there who would use the term "Carolina Kid" passive-aggressively than there used to be.