I’ve noticed that once the GPU has been switched to full frequency from the idle mode (300/100 for GT200) it never goes back regardless the fact that the application that caused the switch is closed long time ago.
This tendency becomes stronger and stronger as long as the driver evolve (I could track the switch to 300/100 Mhz mode back with older drivers that does not occur now).
Why does it happen ? Power consumption and noise are also important things, especially when there are more than one GT200 under the hood.