The restore points clean-up is totally orchestrated via retention policy. Apart from automated retention policy and manual restore point deletion, there is no way how you can decrease the number of restore points.
by Shestakov » Mon Dec 01, 2014 11:42 am 1 person likes this post
Alex, we are always ready to talk with our customers
So, your backup method is forward incremental. It makes Full backup(.vbk file) + chain of incremental backups(.vib files). The thing is, if you delete .vbk file all dependent .vibs will be useless since they contain "deltas" from the full backup file(.vbk). That`s why VBR waits until you have enough "new" restore points after Synthetic full to satisfy the retention policy and then deletes the old chain. You can also find an explanation in the User Guide.