fact mountpoints does not correctly calculates mountpoint capacity

I have (root /) mountpoint my df output looks:

root@test:~# df /dev/sdb2
Filesystem  1K-blocks   Used          Available   Use% Mounted on
/dev/sdb2   19555608   15911760  2627432   86%    /

df displays correct data as 86% used on this mountpoint.

Fact mountpoints displays:

root@test:~# facter mountpoints./

{ available => "3.48 GiB", available_bytes => 3731296256, capacity => "81.37%", device => "/dev/sdb2", filesystem => "ext4", options => [ "rw", "relatime", "errors=remount-ro", "stripe=32624", "data=ordered" ], size => "18.65 GiB", size_bytes => 20024942592, used => "15.17 GiB", used_bytes => 16293646336 }

It displays only 81.37% of used space.

I`ve read df manpage on http://manpages.ubuntu.com/manpages/trusty/man1/df.1posix.html which describes where to get differrent types of spaces and how to calculate used space. As a result I`ve created cpp file which correctly calculates available, free and used space. disk_usage.cpp

 I get output:
fragment size 4096
blocks size in f_frsize units 4888902
free blocks 910962
free blocks for unprivileged users: 656858
total 19555608
available: 2627432
used: 15911760
usedpercent: 85.827690

 

I`ve already created pull request on git to solve this problem https://github.com/puppetlabs/facter/pull/1846