I'm going to guess that the 30" is from the ground to the top of the seat tube, sort of a makeshift standover clearance measurement.
I just went out and measured the woman's Raleigh Sports I picked up this morning (see my his and her's posting). It measures 30" from the ground to the top of the seat tube, and the seat tube itself measures 21" from the center of the crank to the top of the seattube.
Hope this helps.