This is simply the way schedules with non-working days behave. For example:
- suppose an activity on the critical path that is supposed to finish on a Monday is delayed by one day and finishes on the next day (Tuesday) instead.
- Also, suppose that before the delay, the project's finish date was scheduled for a Friday.
Assuming a 5-day workweek where Saturday and Sunday are non-working days, the critical delay of one day will delay the project finish date by one working day, which in this case would be Monday.
In this example, a delay of one day to a critical activity delayed the project's finish date by three days because of the calendar.