The different scores may simply reflect that the national curriculum is more demanding for writing.
Please remember that in the first year of the key stage assessments one piece of research tried both the KS2 tests and the KS3 tests for maths on a group of pupils, and found that it was easier to get a level 4 at KS3 than KS2. There is nothing like sufficient reliability about these measurements to draw sweeping conclusions.
Michael Hurdle. Primary deputy headteacher. 7 Farm Lane, Send. Woking, Surrey