Hi Jitin Joy,
As per system standard it calculated correctly. Since, you are using backward scheduling, for this, you no need to enter start date, you have just maintain finish date. System will automatically calculate the start date based on your in-house production days which you maintained against this material in Material master.If you have use forward scheduling, you have to enter start date, system will do forward scheduling and will find the finish date. The above calculation are used when you generate production order through MRP. If you create production order through manually (CO01) system will not consider in-house production days but will consider the timing which you have mentioned in routing.
Thanks