The retention policy of vRanger is designed for the user to be able to specify at a minimum how many days they want to ensure they have backups for.
The default for vRanger is seven save points (backup points) and when differentials are chosen it defaults to six. This would result in your getting one full and six differential images. You will also see additional images get added until the first set is replaced and then vRanger will delete the old.
If you want to have thirty days of backups at a minimum of a VM then you would want to set the job to thirty save points and you can choose to have a certain number of those be differentials or incrementals.
**Note** When differentials are chosen you can also allow for a full to be run instead of a differential if more than a certain percentage of a VM’s contents change since its last image.