From the DOE Performance Tables:
Demography / intake:
Graveney: Low attainers 9%
Middle Attainers 38%
High Attainers 53% (and 13.9 FSM)
Dunraven: Low attainers: 13%
Middle Attainers: 54%
High Attainers: 26% (and 24% FSM)
So, in a school with more than twice as high a % of high attainers at Graveney (and a big discrepancy in FSM), these are the overall average comparative GCSE A-C inc Eng&Maths for the school as a whole
Graveney: 2011: 77%
2012: 79%
2013: 75%
Dunraven: 2011: 73%
2012: 72%
2013: 75%
Comparing like for like (i.e attainer group against attainer group) the % getting 5GCSE inc maths and Eng:
Graveney - middle attainers: 63%
high attainers: 96%
Dunrave-: middle attainers: 76%
high attainers: 98%
Average score per qualification:
Graveney- High Attainers:A-
Middle Attainers: C+
Dunraven -High Attainers: B+
Middle Attainers: C+
So, on this comparator Graveney gets a better score for it's high attainers - but those high attainers represent a super-selective intake. It gets the same score for middle attainers, where you would expect the groups to be more comparable.
So, yes, I would say that on all these figures, if you remove the impact of the super-selective stream, Dunraven's results are equal to Graveney's. Charter too (because they have better results than Dunraven, I think, but a lower FSM %)- but I haven't got time to demonstrate.
Why should this be surprising?