Hi! With EBS volumes you are not charged for the space you use inside the volume, - you are charged for the space you provision. You mentioned you provisioned 20GB volume. It means that you will be charged for 20GB of EBS storage for the full month (pricing unit = Gb/month).
In your example, I believe you are looking at the billing console of current month (bill for Nov-2022), which is not a finalized bill yet for November. Let's assume that you had your volume available at the very beginning of the month, and let's say you looked at the bill yesterday (23th Nov), which means that you likely saw the bill for only first 22 days of the month. In that case, for full November you would get charged for all 20GB of your provisioned volume. However, for 22 days of the month, you would only get charges for (20GB / 30days * 22days) = approx. 14GB
You can find more info about EBS pricing and how you are charged here: https://aws.amazon.com/ebs/pricing/