Why does Lockbox leave a few pennies unapplied when receipt amount matches invoice?

We have a situation where, only occasionally, the Lockbox leaves a few pennies unapplied when the receipt matches the invoice.  For example, we had an invoice for $50,000 and the customer paid $50,000.   Lockbox is set to match to the invoice number, and it applied $49,999.72   The remaining $0.28 is far too small to be anything like a discount.  The invoice and payment are the same currency, so its not exchange, either.  This happens a couple of times per month, seemingly at random.

Has anyone had this issue?  Or have any thoughts on what to check?

Thanks!

J