IIRC Linux reserves a small portion of every partition (file system) for system use to be able to use if needed for bad blocks, etc…
I have a 3.0T lvm partition that has 2.1T used and 729G available (or only bout 2.8T usable with 200G system reserved). I would not worry about that apparent discrepancy especially since your post shows numbers when booted as live-user instead of as the regular user on the installed system.
That said though, any file system that is 90% and more full will quickly start having problems with slowdowns and fragmentation as the system tries to find space for new and growing files.
[Unix/Linux]automatically reserves 5% of space for superuser access – meaning regular users and processes won’t be able to use this space (filesystem will report to be 100% full), but root user can still write and troubleshoot. (Unixtutorial)
The article linked above also has examples how to tune the file system to change the default value of 5% to less.