aintnomountain: An example.
Child A has a raw IQ of say 120. Child B's is 110 (these are random figures!).
Child A goes to the local state primary. This may be a normal, straight forward village school, it may be super-academic, it may be super 'socialist' where competition or selection of any kind is discouraged. He sits his 11+ having never clamped eyes on a NVR test in his life.
Child B goes to a prep school that specifically offers 11+ prep as it is in a grammar school catchment thus is a popular option with parents. From year 5, child B is relentlessly tutored, tested, challeneged in the ways and means of 11+ exams. He does past paper after past paper.
They both sit the 11+. Child A has never seen an 11+ paper before. He hasn't been tutored in pattern recognition; 30 ways of number or letter sequencing; the finite different ways the same pattern can be turned but still remain the same pattern, etc. He knows no 'tricks of the trade'- he's never actually had to sit down and do a 2 hour exam before! Child B, on the other hand, being reasonably bright and having been 'well-schooled', instantly spots the tricks and can work his way through many different scenarios to solve that sequence.
The grammar has 100 places. A line has to be drawn somewhere. If that line is drawn between child A and child B, B may get in whereas A doesn't. Child B's parents have effectively bought B's place AND deprived A of his. THAT'S why it isn't fair, tax paying or otherwise.
One MIGHT try and say you can't tutor for 11+; it only measures raw talent. Those who have been through 'the system' know differently which is why they (we?!) are prepared to pay out 8K a year for 2 years to get our child into a grammar.