A couple of thoughts on your issue. Since there is no report on the standard reports menu called "AP Summary", there is no way to know how the report you're using is constructed. First, confirm that all GL Transactions are posted. Sometimes changes to AP invoices, including voids, do not automatically post in GL. In GL, go to Tasks > Post Entries and review the output journal to see if any entries are rejecting. The more likely suspect for your issue is that the AP report you're using doesn't recognize an accounting date cut-off. Try running "Open Invoices with Cut-off Date" or "Invoice Aging by Job". Both of those standard reports use the AP Transaction File to determine the balances to report as of the cut-off date. You're likely using a report that is just pulling from the Invoice records, and the voided invoice is no longer included since it's been deleted.

The AP Summary report has some issues, particularly when invoices have been deleted (see knowledgebase article 49937). It is better to use AP Reports > GL Reconciliation > Period End Report for AP Accts.