I was wondering the same thing some time ago.
It’s not a bug, it’s a feature 
du -sh /var/lib/docker/overlay2
is not showing objective value because merge folders have been mounted using overlay driver and du output is not actual disk allocation size.
You can see the actual disk allocation size by examining only diff folders like:
du -shc /var/lib/docker/overlay2/*/diff
You can test this in your environment like this:
run
df -h /dev/sd*
du -shc /var/lib/docker/overlay2/*/diff
du -sh /var/lib/docker/overlay2
Now start 20 centos containers and observe what has change:
for i in {1..20}; do docker run -itd centos bash; done
df -h /dev/sd*
du -shc /var/lib/docker/overlay2/*/diff
du -sh /var/lib/docker/overlay2
You can see that the actual disk allocation (df command) is just cca 200MB more than before, but “du” on whole folder outputs 4.2G allocation.
“du” on “diff” folders shows 212M what is correct.
This is how Docker works and what makes it great!