That's Device Monitor(2), I use it on my iPad.
What you're seeing is the monitor considering both used, inactive and compressed memory in the total "RAM Used" diagram, and inactive and compressed are basically fancy terms to mean not being used! (Wired is what's required by the OS, so it isn't free)
So, basically, take the "total" stat and subtract it by the "inactive" and "compressed" stats, to find you have a nice and average 3.1GB / 8GB of ram actually being used!
There's also an option in that app to only consider "Active" and "Wired" ram to be shown in the RAM graph, meaning you'd get a much more accurate reading. Hope this helps. :3