[Libguestfs] virtdf outputs on host differs from df in guest

hi guys,   happy new year!!!  I encountered  a problem when we used virt-df command to display my guest disks useage. the virt-df command from pkg libguestfs-tools-c-1.36.3-6.el7_4.3.x86_64,version is  virt-df --version : virt-df 1.36.3rhel=7,release=6.el7_4.3,libvirt the output in guest with the method referred in http://libguestfs.org/virt-df.1.html#STATVFS NUMBERS is: [root@localhost ~]# df Filesystem              1K-blocks    Used Available Use% Mounted on /dev/mapper/centos-root  23544596 4335172  19209424  19% / devtmpfs                   931220       0    931220   0% /dev tmpfs                      941860       0    941860   0% /dev/shm tmpfs                      941860    8612    933248   1% /run tmpfs                      941860       0    941860   0% /sys/fs/cgroup /dev/sda1                  508588  133328    375260  27% /boot tmpfs                      188376       0    188376   0% /run/user/0 using scripts: python -c 'import os; s = os.statvfs ("/"); print s' posix.statvfs_result(f_bsize=4096, f_frsize=4096, f_blocks=5886149, f_bfree=4802342, f_bavail=4802342, f_files=23556096, f_ffree=23435372, f_favail=23435372, f_flag=4096, f_namemax=255) python -c 'import os; s = os.statvfs ("/boot"); print s' posix.statvfs_result(f_bsize=4096, f_frsize=4096, f_blocks=127147, f_bfree=93815, f_bavail=93815, f_files=512000, f_ffree=511626, f_favail=511626, f_flag=4096, f_namemax=255) and then on host using virt-df showing: # sudo virt-df rpm-build-for-7.2 Filesystem                           1K-blocks       Used Available  Use% rpm-build-for-7.2:/dev/sda1             508588     107728 400860   22% rpm-build-for-7.2:/dev/centos/root    23544596    4298384 19246212   19% using scripts: # guestfish --ro -d rpm-build-for-7.2 -i statvfs / bsize: 4096 frsize: 4096 blocks: 5886149 bfree: 4810534 bavail: 4810534 files: 23556096 ffree: 23435372 favail: 23435372 fsid: 64513 flag: 4097 namemax: 255 # sudo guestfish --ro -d rpm-build-for-7.2 -i statvfs /boot bsize: 4096 frsize: 4096 blocks: 127147 bfree: 100215 bavail: 100215 files: 512000 ffree: 511626 favail: 511626 fsid: 2049 flag: 4097 namemax: 255 we can saw that in guest the statvfs get the value was the same with the "df" command output. but the bfree/bavail values are different between using guestfish statvfs on guest and using statvfs in guest. I want to know whether is this a bug? or Does same reason cause this appearance？ Thanks, Chen