Service call report-overdue column calculation

IN service report I want to see service call report .There is a column name as overdue as per my understanding this is calculation wrong.

As i have loged and closed the service call within response & resolution time mentioned in service contract even then it is marked tick on overdue.

the call which have no contract it is only showing without tick on overdue in service call report.

how it is possible .

anyone can let me know in which basis of calculation overdue mark tick or without tick.