I would suggest rather than decreasing the reserved block percentage, use tune2fs -u to specify a user id (usually user id of the application which will use the file system, I'm assuming it's oracle...
Also, the file systems leave a small bit of space "unavailable" on purpose. This allows for the root user to access and perform functions in a file system that shows 100% full and not have a full...
Every one of the "real" disk devices shows the same thing. Short answer is that the filesystem uses disk space on the disk that is not reported as used by the file system.
This is normal behavior. ...