The true ability of DCs reflects in different forms.
Selective schools (esp private) can use different selection criteria to assess and select the pupils they prefer. Even most of the selection process is academic based, the exam format can vary with different focus.
For example, SPGS round 2 & London consortium has problem solving questions. NLCS had emphasis on open maths questions. School does CEM select focus on speed and time control ability. HBS had heavy focus at NVR and English. Some other school look for team work and collaboration in the classroom settings.
It is a brutal process, and I think 11 is a bit too early (well then you have 4+ 7+ prep...). But I think if one has to send their DCs to 11+ exam for whatever reasons (mostly academic reason I presume). Different schools having different form of exams is way better than one single exam fits all case. It kinds of smoothing out the variability and give DCs the chances to find the schools that fits their true ability or potential.