I don't know. Harmon is a very good course, and great private courses always get a bump in their ratings. The back 9 is one of the wildest sets of 9 I've ever played: I'm not sure how well-designed it is, but it's interesting enough, memorable enough, and executed competently enough that I can absolutely see that propping the course a bit. There's some good elevation and lots of variety in terrain.
Don't get me wrong: Harmon Hills is barely even in my top ten, so I don't consider it one of the ten best courses out there. But I'm not surprised it is where it is. I think it's clearly better than Fairfield and Lake Claiborne, for instance, which have been ranked in similar positions albeit for shorter periods of time (I think).