About the word 'couple' in the phrase, "a couple of..."?

As I understand it, the word 'couple' describes an amount of two, but most people, including myself, constantly use the phrase "a couple of..." to usually describe or indicate to an amount greater than two. For example: Let's just say John visits his sickly father in the hospital a consistent 3 times a week. When John was asked by his friend Mike how often he visits his father, he said, "Oh, I visit him a couple of times a week."

What I'm getting at is that he could have said, "Oh, I visit him a few times a week." Indicating more than 2 times. Although anybody should understand that when John said "a couple of..." he was just generalizing an amount not too great, I'm just curious as to whether or not it is grammatically incorrect to use that phrase considering the definition of the word 'couple' refers to two.