Under “Configure Backup Contents And Schedule”, the boilerplate text reads “and retain this many SCHEDULED backups:”. But doesn’t the number entered there actually control how many backups are retained regardless of whether they’re scheduled or run manually?
The count doesn’t include automatic backups taken through the backup-before-plugin(/theme/core)-update mechanism (since if someone updates 10 plugins in a day, they don’t want to then find that 10 of their weekly backups got pruned). However, you’re right – it does include the ones created through hitting “Backup Now” too. So, the wording could be improved. I am not sure of a succinct description that can be used, and will need to think about it!