Maybe that's why! People in the lower end at a top school may be less-motivated and relying on the Name Brand to get them a job, while someone at the top of a lower-rated school might have worked harder in school and keep that up afterwards.
Or maybe Vault is just messing around so people can say, "Wow, School X must really be pretty good! Screw Duke, I'm going there! Thanks Vault!" After all, I'm sure those aren't the full extent of comments they got.