Every backup listed as deleted incorrectly was less than 1 day from the previous one, thus was deleted according to the 1 day retention. 1 day is 24 hours or 1440 minutes or 86400 seconds. You can give the interval in those units with h m s respectively. Everything is converted to seconds. It’s not calendar time.
Custom backup retention unexpectedly deleted a backup says more. Use logging to see the processing.
I think it’s backup-to-backup time distance, with backups known by the usual time on Restore dropdown. Deletions of any sort happen after the backup, but the backup just made is exempt from some deletions.