Good question.
No, it's not an American thing for an American to lie as Joe does. But, I suspect he wants to remind EVERYONE that, as Chris Plante frequently says with an Italian accent, "his a'brain, she's a'broke."
Why would he do that?
To prepare us to attribute SOME as yet unknown God- awful thing to his mental impairment OR AN ACT OF GOD, or an accident or freaky nature thing and not to blame them as having orchestrated this upcoming evilness.
'Hey, don't look at me, it's not MY FAULT! My brain is broke, remember?'