Why is it that most self-proclaimed "Grammar Nazis" don't know crap about grammar?

One of my biggest pet peeves is someone going out of his way to correct someone else's grammar, and not only being a dick about it, but being utterly wrong. I just had to listen to a guy patronize a colleague for saying "If I were." He laughed at her and said, "It's 'if I was.' Don't you know that the word 'were' is plural? Go back to grammar school."

No, you go back to grammar school, jackass. "If I were" is correct. It's called past subjunctive. And even after I defended her and explained this to him, he still blindly insisted that he was right.

I have a low tolerance for imbecility like this.