As your weekly, fortnightly or monthly budgets represents an average cost, there will be times when you under spend and then over spend your budgets.  Once you reach the end of your lease if there is a surplus in the budget the funds are returned to your employer and the appropriate tax deductions made and the net balance returned to you via a salary payment.
If there is consistent under spending against your budgets we may need to reduce your future budgets and therefore reduce you future salary deductions.