This article describes a situation where instances show the storage amount on the subscription page, but it is one to three percent greater than the total of the datasets present.
You can see on the image below that the total of datasets is 23.7 GB but the page displays a total of 24.1 GB.
The system file adds the additional storage amount, as it normally shows on any operating system, this will be adjusted so there is no additional cost.
The more data there is, the more noticeable it is.