Earning my No-Prize here, I think that, by the time Bruce got to the point of claiming to know how much the guy had cheated on his income taxes, he was clearly just bullshitting him, but by that point, he'd already obtained enough other information that he could convince the guy that his bullshit was real. Plus, I suspect Bruce was making the leap of logic that, if this guy beat his kid, then he cheated on his taxes, too, based on ... well, I don't know, is that part of the same personality type?