The two findings are, first, the differences between departmental results which are not explained by "difficulty" of different subjects or a "weak" cohort and, second, the lack of difference within departments between better and markedly less good teachers.
This suggests to me a strong departmental effect arising possibly from better or worse leadership at this level. The A-level results probably reflect more the prowess of individual teachers within departments. An analysis at key stage 3 national tests would be interesting.
M F Earley Headmaster St Gregory's RC High School Donnington Road, Kenton Middlesex