This query concerns off roll students. The Census is only looking at students who are between 5 years and 15 years on 31st August prior to that academic year. To check this query, look at the age of students who have left in the period in question (e.g. Spring Census it will be from September to December).
To check this query, you can use Reporting>Reports>Students (Blue Tab)>Admissions and Leavers Report. Census is only looking at students who are between 5 years and 15 years on 31st August prior to that academic year.
Or go to Admin>Students>Search Not On Roll and check to see if there are any student leavers the previous term. Check the age of any leavers, and any were less than 5 years old, and greater than 15 years, on the 31st August will not be counted in the census calculations.
If you can confirm that no students in the age group in question have left the school in the term before this census, you just need to annotate the Census file when you upload to Collect.