I am not sure of the answer with regards to how the show's creators would explain the circumstance. I do, however, have a thought of my own.
As a werewolf and vampire, George and Mitchell (respectively) have their 'foot in the door' so to speak with the supernatural realm. Both men had to experience death in order to attain their new lives. As I understand it, George "dies" once a month. In any case their physiological realities differ from those of ordinary humans. Ordinary humans cannot touch or see ghosts; this does not necessarily mean ghosts are not visible or solid.
This is how I explain the situation to myself :)
