It is possible that you have a transaction with a future date, perhaps due to a keying error, like 10 instead of 01. This transaction would not be included in a report calculated back to a specific date, but would be included in any report using "aged as of".

Although you are correct to use the Aged Trial Balance report calculated to the previous month end, it is a good idea to also print (or preview) a summary report using "aged as of" at month end to catch this type of error.

To correct a date error:
Credit the original invoice using the original "wrong" date for the credit note.
Re-enter the invoice with a similar but unique Invoice number (add A at the end) and the correct date.