Please note that Total Memory in the graphs referring to the memory of the entire server, the memory Wowza is using is the Wowza Heap.
You could try getting a process output and see what processes are running on the server.
Under Linux if you access a file then it will store some/all of it in memory if it is available to do so, only when memory is needed it removes it.
You can see here that 4525664 is used as cached space.
: total used free shared buffers cached
Mem: 10225612 5122476 5103136 0 157464 4525664
-/+ buffers/cache: 439348 9786264
Swap: 2097148 66828 2030320
As this is being used Wowza will show this in the Total Memory as used.