GP's are more difficult and very expensive to compete in, which is why there is a higher proportion of AA grades who are competing for what is a a national ranking and are therefore prepared to devote the time and money travelling to most if not all of the GP's.
I would guess that the lower grades reflect a higher proportion of competitors local to the area and there are proportionally less A/B/C's attending all / most of the GP's.
Changing the grading system based on the grades attending a GP is flawed as it will always be the complete opposite of the normal distribution you get at a local shoot and less so at a regional shoot I would guess.
Make GP's more difficult than they are now and you run the risk of putting off people like me who are really keeping the entry cost for the AA competitors down and ensuring the host club doesn't make a loss if only 75 people turn up, but if too many people start clearing courses then the level of difficulty needs to be tweaked but carefully.
The grading system seems to work based on the scores that are posted which generally show only a small percentage of crossover in scores between the different grades.