Personally I don't understand the grading system at all. The A/B/C category seems to be used by people to indicate a tough race, one where safety gear is necessary, or that it's championship material - none of which are indicated by the amount of ascent / mile. Further, the S/M/L doesn't indicate how long the race is since it ignores ascent (e.g. some incredibly long Lake District AMs).
Personally I'd have rated races S/M/L according to Naismith - #miles + 1.5*thousands of feet ascent (very close to the old record time based system) - then used A/B/C to indicate severity/roughness/navigation involved/seriousness, with A=kit reqd, nav skills needed, or experience required, with C=any one can have a go.
Then 3 Peaks, The Ben, and Edale are all As... but so is (as should be) The Roaches by virtue of being held mid winter and needing kit!