...dont ask me why but 14" wheels actually measure about 15 1/2" diametre rim to rim so 15" wheels will measure approx. 16 1/2". I think it has something to do with the bead of the tyre where it seats inside the wheel, that is where a 14" wheel measures 14" if you know what I mean.
Maybe the seller has just measured them rim to rim and wrongly assumed they are 15".

Jim.