Encumbrance accounting in unmatched Invoices Payables

Hello experts!

I'm using Budgetary Control and encumbrance accounting in Payables. I have read that Payables creates an encumbrance for the expense during Approval, and reverses this encumbrance during posting for unmatched Invoices.

When I approve the Invoice the system creates the following Encumbrance entries:

- Reserve for Encumbrance "01.8321.990000"                                     DR           100

- Item Expense "01.8240.002000"                                                       CR                          100

When I post the Invoice The following entries are made:

- Item Expense "01.8240.002000"                                                       DR           100

- Reserve for Encumbrance "01.8240.999999"                                     CR                          100

I know where the system is taking the Encumbrance account at Approval time (01.8321.990000). What I don't get is why the system is using another encumbrance account (01.8240.999999)  when posting the transaction instead of just making the Reversal using the original encumbrance account.