This value is obtained from this command:
findmnt -Ufnro USED -M /
That should be correct, but you can compare with df -h /. You have a ~1 GiB swap file which adds to this size.
You can run the following to identify where the major used space is coming from:
G_TREESIZE /
Since it is an F2FS rootfs, it is not one of our official images, is it? Or is it the one for RPi we had among our downloads a while ago?