Hello and welcome @Shukrati 
The reports are based on the current data in the database at the time you run one. So, if a user who joined in May was later deleted then they’ll no longer be in the data when you run the report in the future.
You should be able to see those missing 3 users in the staff logs (/admin/logs/staff_action_logs) if you filter for ‘delete user` and have a peek in the ‘show’ button for more details.
For a bit more info, you can see the sql version of the query in this topic: Dashboard Report - Signups