Closed Purchase Order and Did Budget Checking - Encumbrance became Negative Why?

Encumbrance Amount became Negative after I closed the Purchase Order.  The negative amount represents the Vouchers that have already been matched and paid and referenced  the now closed Purchase Order.
I thought the Encumbrance would = zero.  Ie.. the Monetary_Amount on the KK_LIQUIDATION table becomes zero.  The monetary_amount, from what I understand, represents the open balance.

Is this correct?  I am missing a step.