Seb - You claim to know so much about music.

Seriously, if you can't understand this, I think it's you that will lose credibility.
When you're in a studio, you can do take after take. You're basically standing in one place, not running around all full of adrenalin like you are in concert. Naturally, your voice isn't going to have the same strength live as it does on a disc.

I've watched online sessions of the band recording the albums, and I don't see that they alter Jon's voice at all.