The memory usage reported in cgroup differs from the free command

Why would the free command be reporting a much different mem usage number vs cgroup?

$ free -b 
              total        used        free      shared  buff/cache   available
Mem:     2096914432   520863744  1112170496    35389440   463880192  1356828672
Swap:    2145382400           0  2145382400

$ cat /sys/fs/cgroup/memory/memory.usage_in_bytes 
857239552

The documentation for cgroups says that memory.usage_in_bytes is a "fuzz value". I'm guessing that just means it reports an estimate that's close to the actual value. Even if it's an estimate, I don't think it should be this far off.

I'm running Linux Mint 18.2 in a VirutalBox VM.

memory.usage_in_bytes and memory.max_usage_in_bytes report RSS+CACHE, so in terms of your free output:

memory.usage_in_bytes ~= free.used + free.(buff/cache) - (buff)

You may also be interested in /sys/fs/cgroup/memory/memory.stat, though I still couldn't find/compute the free.used value from the /sys/fs/cgroup/memory/memory.stat output.