The clock level of the GPU is based on a number of things besides whether the PC is in a Windows idle state, including:
a) Number of monitors attached - multiple displays require higher GPU bandwidth which can result in higher clock levels. Try configuring only a single monitor and see if clocks drop down to idle levels.
b) 3D Applications - some applications may not appear to be 3D but in fact are making DirectX/OpenGL/CUDA calls to the GPU and can cause the GPU to operate at higher clock levels. These applications can include even control panels and utilities. Try closing all applications on the desktop and see if clocks drop down to idle levels.