This can happen on occasion because of how the Partial pay* field on the loan account record works.  For members who make weekly deposits toward a monthly payment due, once in a while, when the calendar weeks fall just right, that weekly amount might accumulate in Partial pay to the point where a small payment made early in the period, coupled with some leftover money in the Partial pay field from the last period, is just enough to bump the due date forward a period.  Remember that a "period" in this case runs from due date to due date, not a calendar month.

*Refer to the online help links below for a more detailed description of the Partial pay field on the loan account record and the Single payment per period loan category configuration flag.