Most people will have revised less than they should, which is why TSR is flooded with threads from people who feel they did poorly at GCSE but are now determined to make up for it as AS/A2. People fall short of their ability all the time at the GCSEs if they fail to take them seriously, are still a bit immature, go to a bad school, are just plain lazy etc. Then again, anyone of decent intelligence should be able to get 5A*-C passes unless their school was literally falling apart.
Of course this doesn't apply to the great masses of people on here with A*s coming out of their ears. I for one only have two to rub together.