My answer to that question is ...
Apart from the obvious i.e. the kids feel the tension - so there is stress.
The number of entrants - increases or decreases competition and so on ....
I pulled the following together from the data I have available for 2012 exams
Y = -7.5654 * X + 1571.2 Actual exam 2012
Y = -6.4375 * X + 1340 Pre-test 2012 (statement by the parents association)
Y = Awarded position
X = Exam score
On gradient alone it looks like a steeper graph for the actual 2012 exam to the 310 pupil cut off.
Put another way: -
160 was the mark stated (Parents Association) as likely to gain entry on the pre test.
167.45 seems to be the mark (310 places) likely to gain entry based on the 2012 exam alone (apart from LEA factors!!!).
Not much? - look at the equation again and given the 2012 figure that each place increments around 0.1333 points (including age correction factoring).
If at the border you would have had to increase your effective place position by 55 places to stay still, surely not!
Now factor in the LEA rules, some sitting exams at multiple schools, some choosing a different school, children in care and so on.
My previous observations of around 60 places drift would seem to be reasonable and would then fit with the Parents Association indication on the pre test letter.
Confused, I suspect the difficulty levels are very much the same. Certainly the work scoring that we had was absolutely spot on with the Exam results obtained.