Why does stat report no space on / (root) but space available on /system?
stat reports 0 space/blocks on / because 
/ = ramdisk which lives in ram, ram is not a block device.
Your actual ramdrive lives in storage with your kernel an dt.img
on a specified emmc partition labeled boot .

The combined ramdisk, kernel binary, and dt.img (device tree binary)
are known collectively as your boot.img

The boot image also contains some actual directories [real] that function as placeholders, they are usually /data /dev /proc /system

Also your ramdisk contains the necessary *.rc and *.sh configuration and shell scripts necessary for booting you device and establishing/bringing up your operating system. The ramdisk also contains your device's selinux policy configuration, sepolicy and context files, lastly your ramdisk contains the /sbin directory, which is where the adb daemon lives and usually charger, healthd, and ueventd binaries and any manufacturer specific binaries related to services, usually security. The ramdisk/ramdrive is temporary and is dumped out of memory when you powerdown or reboot, and reloaded to it's designated place in memory when your device boots up again.