Why does HTOP show bigger usage than defined in cgroup 'memory.limit_in_bytes' parameter?

Question

I have defined the memory limit parameter in self-created cgroup test-group:

$ cat /sys/fs/cgroup/memory/test-group/memory.limit_in_bytes

15998976

After it, I'm executing the Python builtin HTTP server through the cgexec:

cgexec -g cpu,memory:test-group python -m http.server 8000

If to view the RES column (Resident size) in htop, there is such a dump:

PRI NI VIRT RES SHR

20 0 126M 18412 9848 S

18412Kb = 18.412Mb, but I've defined:

15998976 bytes in memory limit parameter, and it's about 15.998976 Mb

So... Why does htop show 18 Mb, if the limit is 16 Mb?

PS: if you ask me about swap, so there is a dump:

$ cat /sys/fs/cgroup/memory/test-group/memory.swappiness

0

$ free -g

total used free shared buff/cache available

Mem: 15 4 3 1 6 9

Swap: 0 0 0