Would you not expect there to be a difference in the top and bottom of the fretboard that is equal to the thickness of the board?
In other words, the arc of the top of the fretboard should share the same centre as the bottom arc. The way you are describing it with 7.25" top and bottom, it wouldn't look obvious across that short segment of the two circles but it would if you extended the arc of the two circles. l They'd intersect eventually.
Shouldn't it be 7.25" for the top radius and 7.25" minus the thickness of the board for the bottom radius?