User has set up standard working hours per week for the employees and checked the Automatic Overtime option in Employees > Dates & Hours screen. However, some of the time entries of the employees are automatically flagged as Overtime (OT) even if they did not exceed the Standard Hours per week.
This can happen if your employees have exceeded the Standard Hours per day on some days in the week without exceeding the Standard Hours per week. When you set up automatic overtime, Core evaluates the logged time entries against both the daily and weekly standard hours, and flags the time as Overtime when either of the two is exceeded.
If you want the automatic overtime evaluation to trigger only when the Standard Hours per week are exceeded, you should set the Standard Hours per day to zero. This makes Core ignore the daily Standard Hours and it only evaluates the entries for overtime against the weekly Standard Hours. It then flags the time entries as Overtime only when the weekly Standard Hours are exceeded. Check the Core Help for details.