Oh, I wasn't thinking about the maths/science bias of the school per se. I'm more interested in how the grammars and the indies make their selections given the different parameters in both sectors.
The independents have a smaller pool to select from, and are therefore forced to take a wider range of ability, and so they need to ensure that they have boys who will work well in their environments. They can also charge boys to sit their tests. (I wonder if the day is coming when an additional fee will be levied to sit for scholarships?!)
The grammars can only select by test results, but the cost of testing comes directly from their budgets - there is no central funding for this. Off the top of my head a company such as GL charge over £20 per multiple paper to mark and standardise it. Where the marking has to be done by hand eg essays and standard form maths, the costs increase further. Hence the move to sharing a single screening/eligibility test, thereby allowing the schools to either reduce the costs or to concentrate on the more time consuming areas eg writing.
The schools have now been consulting on next year's admissions. Wilson's had already been the school which applied the least weighting to the SET (30%) and most to their own maths and English papers (35% each), with the other 2 schools being 40:30:30 I believe. For next year the Wilson's ratios are 20:40:40, so the English paper will have even greater weight. I'm speculating as to why they've gone in that direction, and my assumption is that they feel that they can differentiate between the boys better via the English than the maths (I get the impression that an awful lot of these boys can get at least 80% in a maths paper every time without breaking a sweat, with many who would feel that 95% would be a disappointment!).
I do think that there are potential dangers if creative writing does become the key differentiator for an intake given it can be very subjective to mark.