It's done by some people intentionally to be offensive, and it's impossible to tell if someone doing it is doing it as an insult or out of ignorance. And, believe it or not, there's enough people out there that do it intentionally that assuming ignorance would be incorrect more often than it'd be correct
The chance you get the gender correct when you know the real name, the voice, the look and the behavior of a person is significantly higher. Thus it makes more sense to raise an eyebrow when someone misgenders in real life.
