In reply to Ramblin dave:
A friend of mine seemed to think the explanation was that if it feels ridiculously hard for the grade, you are probably doing it wrong.
He has spent a lot of time in the forest and has often been scratching his head at a 6a when an old bleausard will come along and indicate that your foot goes 'here' and you aren't allowed to use 'that' hold and suddenly the route makes sense.
Font routes can be incredibly hard to read if you ask me, there's always a clever trick.
This assumes that you mean grades having no correlation only works the one way, easy graded climbs seem impossibly difficult, which has so far been my experience, I rarely find myself cruising up a 6c+ but can often be found staring blankly at a 5b...