Surbiton got 85% A*/B which was impressive given it is somewhere between St Cs and LEH, in terms of selectivity. I assume the larger six form entry intake evens out the usual fluctuations in other private girls' schools results.
There is a actually a lot of swapping around in private school sixth forms. 16 year olds are almost rational adults and the girls' schools in particular can have up to a third of the cohort move on including into state schools and sixth form colleges, for a variety of reasons e.g. for a coed more university like environment, for subject offerings/strength in certain subjects, tutorial group size (there is a minimum number with which you can generate decent A level standard ideas and discussion even if it is economic to run a subject course, losing almost all its sixth form for that reason was what led to the demise of St David's), for economic reasons or some just fancy a change. Subjects like music and art are particularly susceptible to pupils moving to specialist schools/ departments (private schools may have flashy art and music departments with lots of exhibitions and performances to impress parents but academia is still their focus and their conventional approaches are not necessarily the best preparation for those who actually want to go on to the best art /music schools ). There are even some misguided parents who think shifting into the state school at 16 gives an advantage in Oxbridge / university entrance, though unis can see where a pupil was to 16 and wouldn't distinguish between good private and state schools, just between underperforming and good schools.