Past Due Sales Orders show suggested due date earlier than plan run date

We are on 12.1.3 and use unconstrained ASCP plan.

We have noticed that if the ASCP plan is run say today 7-jan-2014 and there are past due sales orders(schedule ship date say in dec-2013) the suggested due date of these sales orders after the plan has completed shows as 6-jan-2014 23:59:00 .

On the other hand if there is a WO for the same part which is past due ,its suggested due date is moved to plan run date and shows as 7-jan-2014

.Is this expcted behavior for past due sales orders to have a suggestd due date as in the example above?If so what is the explanation for this?