In reply to skeev:
It is the same for sport climbs, there are many guidebooks that run trad and sport grades side by side. An example is Millemetre War at cheddar. It gets E6 6b, F7c. It has no gear. If it were soloed I guess E8 6b would be closer the mark. But since it is safe due to bolts, E6 6b just represesnts how sustained/the number of 6b moves.
Whereas, The Aardvark and the Ferret at Avon is a trad rotue and gets E6 6b but is only F6c+ to toprope, this is because it is dagerous. If it were bolted, it would get F6c+ and a corresponding trad grade of something like E3 6b...
The answer is, basically, that if a route is safe, the adjectival grade will only represent how sustained the route is.
That is not very clear sorry!