There should be a 5% difference due to space reserved for use by the root user (and this seems your case, as the difference is exactly 5% of the total space).
However, I have also found reference on some Oracle documents about df
reporting only 90% of maximum available space, the remaining 10% being left as "buffer" for better performances.
Displaying Disk Space Statistics (df)
Use the df command to show the amount of free disk space on each
mounted disk. The usable disk space that is reported by df reflects
only 90 percent of full capacity, as the reporting statistics leave a
10 percent above the total available space. This head room normally
stays empty for better performance.