To be able to restore from a forward incremental backup, you need to have a full backup and a chain of subsequent increments on disk. If you delete a full backup, the whole chain of increments will become useless. In a similar manner, if you delete any increment before the restore point to which you want to roll back, you won’t be able to restore your data (since later increments depend on earlier increments).
For this reason, if you select forward incremental backup, in some days there will be more restore points on disk than specified by your retention policy. Veeam Backup & Replication will remove the full backup chain only after the last increment in the chain meets your retention policy (which will happen once the retention policy reaches the next full backup).
For example, the retention policy is set to three restore points. A full backup is performed on Sunday, incremental backups are performed Monday through Saturday and a synthetic full backup is scheduled on Thursday. Although the retention policy is already breached on Wednesday, the full backup is not deleted because without it the chain of increments would be useless, leaving you without any restore point at all. Veeam Backup & Replication will wait for the next full backup and two increments to be created, and only then delete the whole previous chain consisting of the full backup and increment, which will happen on Saturday.