Dear Trinath
If these two are not the reasons what is mentioned by other colleagues, then I can assume only one reason, that for this order you have calculated and posted WIP earlier and now the deliveries are completed for thsi order and you have to calculate variance on this order.
You are calculating WIP on this order hence the system is calculating WIP amount (i.e. Zero) which should remain on this order as WIP. This is required to reverse the WIP entry already posted earlier for this order.
Regards
Rajneesh Saxena