first off, communicating can also be caused by increased CSF production, but thats less common
secondly, normal pressure hydrocephalus is due to an expansion of the ventricles more than it is a problem with reabsorption.. So basically its like expanding the allowable space for CSF to occupy, so the pressure doesnt increase even though the CSF does.. this is the Wet, Wacky, Wobbly, whatever triad in reversible dementia.