Add to the list of things I just don’t get

Why is it that people who otherwise consider themselves so courteous and considerate to everyone around them have no qualms whatsoever about interrupting some people when they are talking?
Now I’m not talking (well, actually usually I am and this happens to me, which is the whole point behind this posting) about a bunch of people in a bar when everyone is talking at once, or some other equally situation where politeness is not the first consideration. I’m talking about in a two or three person conversation, or, like tonight, at a meeting where for all the other speakers it’s been one person’s turn to speak and then another person’s and the rest of us wait until that person is finished before we start.
My boss does it to me constantly. And usually glosses it over by ‘well, you stopped talking’ ‘No, I was in the middle of a word’ ‘No, you stopped’ and usually I give up because I really am trying to pick my battles. Although occasionally I have walked out of the room or just turned around and started doing something else. Oh, and yeah, this has been going on for years.
But now there’s another person from work who has started to do it. I let it pass when it happened a few days ago. Tonight it happened twice, the first time I kind of let it pass, the second I just sat there when she got done with what she had to say, had to say before I finished what I was saying, and when someone else said ‘what were you saying’, I didn’t respond immediately and the person who had interrupted then said ‘oh she’s made because I interrupted her – again’.
You’re flippin’ well right I was mad, and you making a joke of it and making it seem like it’s my fault because I mind your rudeness just makes me madder. And what I was really doing was trying to control myself so as not to be equally rude and say some thing I’d only be chastised for later (business meeting, the boss was there).
But if she’d’a done it again I probably would have got up and gone over and bit her effing nose off. And I might if she keeps doing it. Except that tonight she had a sudden allergy attack and her nose was running and that would have been just too icky.
But what makes otherwise civilized people do things like that? I just don’t get it.
Add that to my list.
Here’s what I think – rude is rude, and if you’ve been rude at least be polite enough to own up and say ‘sorry’, don’t make it the other person’s fault. I hope it comes back and bites them in the ass some time. Is that rude of me?