Difference in memory measure between top and cgtop

Issue

I've configured systemd to gather memory and cpu consumption for cgroups in /etc/systemd/system.conf

DefaultCPUAccounting=yes
DefaultMemoryAccounting=yes

Reloaded systemd and observed the systemd-cgtop output.

$ systemd-cgtop
Path                            Tasks   %CPU   Memory  Input/s Output/s
/system.slice/firewalld.service     1      -    33.1M        -        -

The Memory value is different from the RSS column in top.

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root       663  0.0  0.7 357860 28900 ?        Ssl  09:21   0:00 /usr/bin/python -Es /usr/sbin/firewalld --nofork --nopid

What does systemd-cgtop account in for memory usage?