MIRO invoice reduction balanced amount not zero

Hi Team,

Kindly help as I am getting balanced amount while posting invoice reduction and I am entering the following details:

1) Invoice date

2) Posting date

3) Reference

4) PO & enter

5) Vendor error

6) Entering the vendor amount same as in vendor invoice (also it is matching with the PO amount 266.25 and tax 20% 53.25)

7) Accepted amount is zero as MIGO is for zero qty. because vendor liability should not open or pending.

8) selection of check box for calculate Tax

9) System calculating 317.50 including tax of 53.25 Balance amount is 317.50 - 319.50 = 1.60

My issue is how this 317.50 is calculating by SAP whereas I have checked manually it should be 319.50 please extend you help.  

With Regards,

Avinash Kowale