Here are a few differences between the studio and the live setting (some things were already pointed out by others), all which contribute greatly to the vocal performance:
- In the studio, James is in a comfortable setting, where he is standing still and can put 100% focus into every line
- He is able to record his vocals in very small parts, so he can focus on each line much better. As for the difficult lines which he may fail a few times, he is repeating them over and over, so he is practicing them with each take. A luxury he does not have at a show
- The post-production engineering can be used to fix any mistake. Small amounts of auto-tune are used to fine-tune any lines which may be off a bit. On top of this, they choose only the best takes to begin with....ones which require very minimal processing
- Professional mixers use compression, reverbs and delays, among other effects, to achieve an optimal sound. The end result is the best possible vocals
As for James live:
- He has to perform every single song, line after line, in succession. His vocal chords become worn out as the show progresses
- He is walking around on stage, in a very high-energy setting. He needs to focus on being a showman to some degree, something he doesn't need to do in the studio
- On tour, he is performing show after show, night after night. There is a high possibility that he will not be "on" every single night. Also, his vocal chords can become worn out from so many shows
- He only has one chance to do everything right, without any breaks