This would indicate there is a new and/or future student that has been manually added to the system.
1) Firstly, please check in the Admissions area via Admin > Students to check if the new and/or future student is there. Then, ensure that you set the student's admissions status as 'Enrolled' so that they pull through to this report.
2) Secondly, please assign the student to a form via the Core Information area in the extended tab and click the 'Save' button at the bottom.
When you run the Class Numbers report again, you will find that the total number of students in the year is equal to the sum of the total number of students in each form.