NotaDisneyMum
OK - Imagine you are 11, and score 70 in the reading test. This means your reading age is the same as an average 7/8 year old. (For every 10 points you are below 100, you knock off a year)
KS2 Maths test papers include quite challenging words - appropriate for an average 11 year old (who will have been taught what the word "hypotenuse" means and knows what it looks like).
Why should a child who might be able at Maths, but not at reading, be unable to score well on a Maths test, simply because they cannot read/understand all of the words?
That is the thinking behind it anyway, whether people agree or not. It used to happen in KS2 Science tests as well, before they were ditched.
I have to say, I had no idea this happened until I started working in a school some 8 years ago. Many children we have are capable of getting a Level 4 or even 5 in Maths, but only say a Level 2 or 3 in English. With someone else reading the questions to them, the child is then able to apply the mathematical knowledge they have, without any detriment because of their poor reading ability.
English comprehension is tested separately.