This occurs when repositories are backed by storage that has the ability to reduce the space used by restore points, like deduplication storage or storage that is compatible with Fast Clone.
The 'Used Space' value is calculated by Veeam Backup & Replication using the size it recorded as having been transferred for each restore point associated with the repository.
It is an expected possibility for the 'Used Space' to be greater than the repository's capacity in environments using Deduplicating Storage or Fast Clone compatible storage (XFS or ReFS).
However, if the environment is not using either of those technologies, please collect diagnostics logs and open a case with Veeam support.
This form is only for KB Feedback/Suggestions, if you need help with the software open a support case