You are asking why Xserver never goes idle while you are using Xserver to run a program to monitor Xserver.
In case you have problems understanding @vi_'s response: this is like saying "why is my display using power while I use it?" When the N900 is idle and locked, Xorg sleeps as well Advice: if you want to look at what goes on on the N900, the best is to keep it locked and login per ssh. There you can use ps, top, powertop, etc. on a system that is "as idle as possible"