"More accurate disk percent usage" #157 change produces strange numbers

[VeselaHouba]

I appreciate @SaviourSelf work #157 as now the warning comes before it's too late, but we found some strange numbers:
In version 5.1.2

CheckDisk CRITICAL: /backup 100.14% bytes usage (10.46 TiB/10.55 TiB)

Old version 5.1.1

CheckDisk WARNING: /backup 99.14% bytes usage (10.46 TiB/10.55 TiB)

Check definition

check-disk-usage.rb -r -w 75 -c 90 -l 20 -m 0.8 -x proc,tmpfs,aufs,nsfs,debugfs,tracefs,nfs,nfs4 -p '/var/lib/docker/'

df numbers

# df -h /backup/
Filesystem              Size  Used Avail Use% Mounted on
/dev/mapper/vg0-backup   11T   11T   94G 100% /backup