I'm reading PMBOK regarding the Earn value session, there have a sentence said

" The EV being measured must be related to the PV baseline (PMB), and the EV measured cannot be greater than the authorized PV budget for a compnent."

My concept is If EV greater than PV, more job is done, but why the EV cannot greater than PV? Anyone can explan to me the concept behind that. Thank you very much.