"It appears that Reading cohort had much higher mean score and smaller standard deviation (probably 15) hence lower standardised score."
Help thickos like me out here - what does this mean?
I just meant that children in Reading Boys result on an average scored more than those in Slough. Also, range of Reading boys scores would be smaller (probably 68-141 like Kendrick and many other schools) than Slough which spread their scores over a bigger range,i.e., from 28-165. Both these factors have caused 16-27 points reduction in Reading scores (of the children I know) when compared to Slough.
Using a smaller range (68-141) has no impact on your chances of getting into the school because it does not affect your percentile rank. However, Reading children scoring higher on average means that there is more competition for the Reading boys places and that means you have a lower percentile rank (% of children who scored below your score) when compared to Slough.