There are no "other contracts" John. I don't know why you would lie bout something so easy to debunk.
At this point, I think John is physically incapable of telling the truth, or at least not without cloaking it in even more lies. Like, I think if you put a gun to his head (after first restraining it so he isn't gurning all over the place) and asked him if he was John Walker Flynt, he still wouldn't admit it.
It's a trait of his that has been baked into his very existence for at least his entire adult life, and probably going back into childhood. He definitely seems the "girlfriend in Canada" or "my dad works at Nintendo" type, the kid that just made stuff up to make himself seem cool but never knew when to stop. Like I've said before, I wouldn't be surprised if any bullying he received was less because he was a queermo and more because he was just a colossally obnoxious retard that got on everyone's nerves.
But we have evidence of his behaviors in adulthood at least. Like
this archived thread from an Ole Miss message board where they discuss lies of his like being a political jet-setter and getting amnesia from being stabbed in the head. Or who could forget his classic dating advice of just lying to your significant other over and over? Clearly this is nothing new for Johnny.
As to why he does this, aside from a lack of impulse control, there's also the fact that nobody really cares enough to debunk him. We're pretty much the only ones that bother, and nobody listens to us anyway (prime example: we've known he's a man for a decade). For a compulsive liar, nothing feels better than thinking they got one over on a rube. John is too stupid to realize how little he matters in the grand scheme of things, so he keeps getting dopamine hits every time he thinks he got away with another lie, when in reality nobody saw the point. Only when he has no other choice does he tell the truth, and even then he'll keep lying about the details; he finally admitted to being a tranny, but he still peddles the adoption lie, for example.
So yeah, John lies because it's all he knows how to do, even about things that are painfully easy to debunk.