Powerbook does not shutdown completely?


This may sound very strange. (A minor bit irritating problem).

Sometimes when I try to shutdown , i.e. push the power button and press Enter to choose the default option (Shutdown), I notice that my 12" Powerbook seems to go into sleep mode (The sleep light is on). However it's not really sleeping. I open the lid and try to wake it. The HD makes a brief sound as if spinning for 2 or 3 seconds and then complete silence - achieves Shutdown.

This doesn't happen if I use the mouse or trackpad to click/select "shutdown". is there something unusual here? Has anybody else experienced this behaviour? Any suggestions?