Why Purchase Orders that are cancelled still add up to the project encumbrance total.

WHY Purchase Orders that are cancelled still add up to the project encumbrance total?

EXAMPLE 1.
1.  07/23/12 A requisition is created with project lines (Req XXX:  1 line for $1,500.00)
2.  07/23/12 requisition XXX is approved.
3.  07/23/12 The requisition sources to a purchase order with project lines (PO YYY: 1 line for $1,500.00)
      This purchase order also has a contract Z

4.  07/24/12 A change order is done cancelling the purchase order (no receipt or voucher activity is on the purchase order).
5.  The PO encumbrance amount remains on the Project Valuation screen and on the Activity Valuation (tab).
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