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Great question! My first guess is that this is related to Linux's habit of reserving 5% of all blocks for the root user (you can read about it here: disk usage - df -h shows incorrect free space - Ask Ubuntu ) but that's just a guess, and regardless of the reason it doesn't SOLVE the issue you're having.
You have a few options to get a more accurate reading.
1) Monitor via the Linux agent
2) Monitor via UnDP (universal device poller)
You can find more info on the UnDP here: FEATURE REQUEST - "Real" Disk statistics in Linux System - Case 870046
...and you can find info on the Linux agent here:
Orion agent configuration on Linux - SolarWinds Worldwide, LLC. Help and Support