Discount Amount Display as Wrong value?

[vipinsimpl4aed]

I did create a Coupon code with a $20 discount.
But When the Customer applies the coupon, it showing the wrong amount. It is not happening every time. But most of the time it was wrong.

My Coupon code Rule Settings.

Websites : main Website

Customer Groups : General

Coupon : Specific

Uses per Coupon : 0

Uses per Customer :1

Priority : 1
Public In RSS Feed : Yes

Condetion : Subtotal  equals or greater than  20  

Actions

Apply: Fixed Amount With Whole Cart

Discount Amoun : 20

Apply to Shipping Amount : NO

Discard subsequent rules : NO

But the Discount is showing : Discount ($20 Coupon) -$20.01

Why this 0.01 variation on the discount?

Please help me to fix this issue