Theres a few differences with live singing - nowadays some people employ an "autotune" which a PC can do to smooth out any blips in pitch. You cant always calculate the right acoustics for the room, and the singer cant always hear the mix of the instruments without proper headphones. You dont go into the studio with 10,000 people staring at you, and as previously mentioned, sometimes some takes are just better than others.
To sing and perform well live really does have to be learnt, its frustrating to go home and do things perfectly then let yourself down in a live situation, but it gets better