presumably the A/AS scores are an indicator of the children that they have at the school + teaching + amount of work. If a children is likely to get 3 Es and goes to a school where everyone else gets 3As then they may do better than they would have done elsewhere but they may be miserable too. Also school may not be keen to take them in the first place.
Conversely, a school which does a fantastic job of getting people with weak GCSEs through good-ish A Levels may not be the right place for someone with strong GCSEs looking to get a bunch of A*s at A2. There's a tacit assumption in CVA that there's a readacross between the two situations, and that "doing better at A2 than you did at GCSE" is the same for different GCSE starting points. But it's not necessarily true. It would be very interesting to see CVA broken down by initial level, but there are few schools large enough to do that in a way which is statistically even remotely significant. Doing an amazing job of getting ESL students with poor KS2 results through 5 GCSEs at C or better will make for a high CVA, but may or may not be relevant to pupils in other situations.