I agree with Mike E.
"A couple of" is used for countable objects and means "two, or possibly three, or conceivably even as many as four, if you're not counting carefully". If you asked someone for "two slices of ham" and they gave you three or four, you'd say "sorry, I only wanted two". However, if you asked someone for "a couple of slices of ham" and they gave you three or even four, you would not say "that's incorrect; I asked for a couple, not three [or four]"; you would rather say "thanks, but that's a little more than I need". "A couple" means "about two, but possibly more if they're small [like slices of ham]".
I would say that "a couple" does not mean "two, but I'm not quite certain", but rather "approximately two". For quantities, "a couple of" it means maybe 1.5 to 3, so "I haven't seen her for a couple of years" could mean maybe 18 to 36 months.
Some might disagree and say that "a couple" might include three, but never four.