Hi,
I got your point. First the order got scheduled at the time of creation. Now this will have basic start and finish date. Again you have changed the scheduling margin key in order and lets say there your float after production was 3 days. Earlier the finish date was lets say 10.3.2013. Now if you schedule again, the date will be 13.3.2013 for the float.
Now again if you revert back the schedule margin key with 000, it will take the finish date as 13.3.2013 and will schedule again. Hence your start date will be also changed. So you are not getting the earlier date. This is for system will always schedule by end date if backward scheduling is there.
Regards,
Krishnendu.