Note that the finding of lower ADH (vasopressin or antidiuretic hormone) goes along with higher rates of excretion, even if it doesn't cross the threshold of polyuria. This is commonly associated with hypovolemia and orthostatic intolerance. This probably affects one patient subgroup more than others.
As for the finding that there were no more significant adverse events in the clonidine group, this reflects either the insensitivity to adverse events of typical experimental protocols or compensation by patients to avoid adverse events when they felt weak or dizzy. Psychological treatments aim directly at eliminating this compensation.