You can read more about standardised scores on the NFER website.

We do our best to standardise the results in our mock exams. We know how people in the past have done in our mock exams and they tell us their standardised score they get in the actual 11+. We can then work out a rough correlation between our scores and the standardised scores in the 11+. So, for example, if we standardise a result of 80% as 105, it means that the average score achieved in the 11+ by children who got 80% in our exam was 105. They probably ranged from 95 to 115.

Our standardised scores are in the midpoint of the likely range that will be achieve. So, if we give you a standardised score of 100 and the pass mark turns out to be 100 then you have a 50% chance of getting 100 or more and passing and a 50% chance of getting less and not passing.

We feel that educational standards in children have dropped this year so rather than relying on correlations from previous years, we've allocated standardised scores more based on the fact that about 75% of students that attend mock exams normally pass the 11+. So a standardised score of 100 in our exam shows you are roughly in the top 75% of children

SalisburyElevenPlus.com