How much memory does my system really use?

I use three methods to monitor memory usage:

1. the following terminal command:

free -m

3. xmobar

I ran them all at the same time, and assumed the output of the three methods would be the same.
Wrong! They're not even close, as shown by the figures below.

The output of free -m shows my system is using 493 MB of RAM.
The output of conky shows my system is using 655 MB of RAM.
The output of xmobar shows my system is using 696 MB of RAM.

Am I doing something wrong? Which method is accurate?
I'm using Ubuntu 16.04 LTS.