Your child would have been included in a standardisation column which related to the candicate Age in years and months on the date the candidate took the test. So it's just possible that taking the test a week early may have flipped him into a younger set than the other candidates who took the test a week later.
So it's is possible that a child with exactly the same birthday as your son's might have been slightly disadvantaged - however in practice the difference would be miminal but they would have had an extra week to prepare!
Age Standardisation varies from Test to Test, but if you want an example of what Age Standardisation has on raw scores take a look at the Standardisation tables at the back of the 2007 Key Stage 2 results
Hope this helps