# dumpe2fs -h /dev/sda5 | grep -iE "reserved|Block size"
dumpe2fs 1.42.12 (29-Aug-2014)
Reserved block count:     175436
Block size:               4096
Reserved GDT blocks:      856
Reserved blocks uid:      0 (user root)
Reserved blocks gid:      0 (group root)

For this mounted partition returned free space will be 5% less (Reserved block count * Block size) than 'total-used' and it's not very obvious for zabbix users.

To improve it, to Comments I'd add a note:
Reserved space of a file system is taken into account when use "free" mode.
https://www.zabbix.com/documentation/2.4/manual/config/items/itemtypes/zabbix_agent

Sasha supposed that if could be related to vfs.fs.inode too, but using the same method as for vfs.fs.size and 2 different partitions (0% and 5% reserves) I could not confirm that.