This happens if the bill-wise details in sales invoice is not altered manually against the advance received amount.
Suppose you have received advance payment from a sundry debtor as on 16.4.2020.
A sales invoice is recorded on 24.4.2020 and adjusted against the advance receipt amount.
In Bill-wise details if the against reference is selected for advance without altering the sales bill amount.
In Ledger Outstandings report, overdue by days will consider the date of advance received and not the date of sales bill.
To calculate overdue by days as per the sales invoice date, you need to mention the advance amount manually, and then allocate the balance amount to new reference
If the amount is allocated correctly in bill-wise details of a sales invoice, then overdue by days will show the correct figure.