I'm not sure how the industry measures "cutting diameter" but sayif on a 3 blade head you just measure from the outermost point on one blade to the outermost point on another blade thatwouldn't be right. I think you have to take all three outermost points then draw an imaginary circle encompassing them. The diameter of that circle is the advertised cutting diameter.

edit: the diameter of that circle will always be bigger than your measurement from blade to blade because of the arch in the circle. Does this have something to do with your measurements coming short?