A couple of things can clearly be two things. For many people though, a couple of things can include three or even four things.
That's because the phrase "a couple of" has some elasticity to it.
Professor Anne Curzan admits that she was once adamant that "a couple of things" always means exactly two things:
"If I started an email and said, 'I have a couple of questions,' and then started listing my questions and discovered that I actually had three questions, I would go back and change it and say, 'I have a few questions.'"
A couple of years ago (or maybe three or four), one of Curzan's students conducted a small survey about this phrase at the University of Michigan. She found that for more than half of the people who responded, "a couple of" could or even must refer to more than two.
Curzan and her student checked usage guides and dictionaries and found that, in fact, "a couple of" is synonymous with "a few" or "several."
"In other words, I stood corrected," Curzan said.
This doesn't mean that "a couple of" can't refer to two things, but it's also, loosely speaking, more than that. If you tell someone you'll call them back in a couple of minutes, do you mean exactly two minutes? Unless you've got your eyes glued to a stopwatch, probably not. You probably just mean a short amount of time, at least two minutes but probably not much more than five.
Please note that "a couple of" is an informal phrase and is best avoided in formal settings or situations where precision matters. In other words, if your dentist or doctor tells you they're going to extract "a couple of" something, it's probably best to ask for clarification.