Scott Catalog lists the value for stamps with a grade of "very fine". What would be an appropriate discount for a stamp with a grade of "fine". As an example Scott #292 the $1.00 Trans-Mississippian mint hinged with a grade of very fine is valued at about $1000. What might a stamp in the same condition be worth if it was graded fine.
I'd also suggest that fine on some of the issues isn't necessarily fine. For example take the 3c 1857, and if you go by the definition of fine the perforation should not touch the design at all. #25 with the entire design clear of the perforations would be a condition Rarity for that stamp. And a #26 is one of the more common of the early issues, but still very difficult to find one when the perfs don't touch any part of the frame.
Bottom line is that this can also lead to pricing guides being confusing.