who has more knowledge

hi all,
i was just wondering how the certifications test a candidates knowledge to the fullest. but is everything studied actually applicable in the real world? and do the knowledge gained by hands on experience is capable in itself in getting one certified? who has more knowledge? like an example, there are two guys A and B. A has suppose two years of job experience and B has studied hard and has enough hands on experience, though in a lab only. can A clear the certification grill without much study or can B equal the real time experience of A with his certifications the and lab experience.