A better question would be why: as soon as your A-levels (or IB) are done, your GCSEs in those subjects do not matter. What does matter is the number that you have, and that you have both English and Maths at a decent grade.
Having AAA at A level is a lot better than having AAB but 2 extra IGCSEs in subjects you have mostly at GCSE anyway. Sure it's possible, but still why!?