chaycock is right. From Veeam's Help Center retention article
Veeam Endpoint Backup retains restore points for the last N days; the number of days is defined by the user. During every backup job session, Veeam Endpoint Backup checks if there is any obsolete restore point in the backup chain. If some restore point is obsolete, it is removed from the chain.
For retention policy settings, Veeam Endpoint Backup takes into account not calendar days but days on which backup files were successfully created.