I have never heard an artist sing better live than on record, in most cases the artists improvise their songs during a concert. In Jimmy's case I feel that he is showing his ownership of his songs by putting some twists onto his music (such as shouting the lyrics or even changing them at times.) This to me is why I like live music rather than tapes (I used to think differently, but I realize it makes certain songs more listenable when you hear them over and over again if they sound somewhat different.)
Myself I know I don't have a great voice, but when doing Karaoke I can do songs like Chattahoochie and Brown Eyed Girl and people applaud. The people are not applauding because of how good I sound, it's more on par with how I express the song through my singing.