Well if you haven't seen enough calculators to see one that can't add, a very common issue with floating point arithmetic on computers, you shouldn't offer your experience as an argument for anything other than that you haven't seen enough calculators.
Well, to make the claim you actually made, which is that you haven't seen a single calculator that was wrong, anywhere from zero to all of them. It's just that the "zero" end of that spectrum doesn't really tell us anything about calculators.
