magento coupon code not apply on grandtotal

I have issue/confusion.

While I try to add a coupon code to my Magento 1 store at that time the Grandtotal (Including Tax) not changing.

Coupon information

  1. Code: demo123
  2. Condition: for all products
  3. Action: 5% of products prices.


enter image description here

While you check in image.

Total = 12.99
And discount = -0.65

So grand total(Include Tax) must be 12.34 but as you see in the image it is showing me wrong.

Tax = 21%

Any idea why this is happens?