You will have set the Date Range in Step 2 out of 8 of the Custom Report to be 'Current Academic Year'. This means that all the students that have an active enrolment at some point this Academic Year will be included in the report.
How do I make my report only include Current Students?
When the report hasn't yet been created
Change the Date Range to say 'Today'.
When the report has already been created
When on the report, click the grey Edit button.
Change the Date Range to 'Today'.
Click the green Save & View Report button.