“I'm not exactly sure,” used to be a pet peeve for me because when someone says that, what they're actually saying is, “I have no Earthly idea, but I don't want to sound like a damn fool.” Just admit that you don't know! Forget “exactly sure”, you're not even close to “sure.” You have no probable hypothesis to be “sure” about! But no one thinks about it. It's one of those phrases that has come to mean something entirely different from what it would mean if taken literally. I always imagine calling them out on it, but I never do.