Complete Durations Extended

Can someone explain what the reason is for tasks that have actual starts and are statused to the data date,their complete durations are increased by an equal amount for all.  The durations of those tasks not started do not change.  In my project each task's complete duration was increased by 10 days.  Other projects increase by 30 days (for each task in progress).  Some schedulers populate the expected finish date which works in some cases but not in all and that should not have to be the case.