But it doesn't necessarily mean two for everyone. As far as I can see, there are some people for whom it always means two, and exactly two, and those people are to be found all over the internet referring to their outdated rule books and invoking etymological arguments. You can ignore those people. There are also people who, similar to me, perhaps, mean around two but maybe three or four. And there are some people who mean three, or four, or a few, or more than two anyway.
I had an encounter when I told someone of this persuasion (with an Irish dialect) that I had a couple of questions. I asked precisely two questions, and after she answered them she waited patiently for the next. When I said that was it, she said Oh, you meant two when you said a couple. For her, a couple definitely meant more than two.
On the TV programme Two and a half men (I like it, OK? It makes me laugh) Charlie Sheen responds to another character saying I only had a couple of [something - drinks, maybe?] with the line,
Jack the Ripper only killed a couple of prostitutes.Jack the Ripper killed at least five women (and they were still women, by the way, despite being prostitutes). I wonder if Charlie sees a couple as always being more than two, or 2-5, or if he was deliberately expanding the meaning of the phrase for the sake of the joke.