It's like saying "I think the probability is 100%" or "50%".

Hello, my name is John, nice to meet you. To answer your question certainty has grades and is not absolute on its own. This is the reason why people as "How certain are you?". This is because I can be certain and yet have doubts about something. For example: "Are you certain you saw someone? Yes I'm fairly certain I did but I can't say for sure." Certainty depends on the person and the situation. I hope this helps.

Weirdly enough, it has become common for people to have conversations like - How certain are you? - Oh, about..... er...... 75% certain. I'm pretty sure, but not completely sure. You're right that you would only say "I'm certain" if you were absolutely certain, but you can also use it in a relative sense. "Half-sure" and "half-certain" are fairly common phrases.

Possibly ... but it is widely used (along with "absolute certainty", "betting certainty", "complete certainty") as are expressions which qualify certainty (e.g. "fairly certain", "reasonably certain"). You can also use phrases like, "almost certainly." "Certain" has in these uses come to be a synonym for "confident", and I guess we don't all speak like logicians!

@Sam, "... and I guess we don't all speak like logicians" yeah; in everyday situations we need not to [Mr. Wittgenstein will disagree]. Though it's not rare that a different interpretation of a single word [often in a religious scriptures] has given birth a whole school of thought or ideology. Mr. Wittgenstein might have a point after all. @Jon, so are those common phrases, strictly speaking, wrong but still common? If they are wrong, then I guess there must be a [very?] good reason why they've found their way into the common use. @John, "... certainty has grades and is not absolute on its own" that was my main doubt. I, for some reason, compared it to "dead". We can't say "a bit dead" or "more dead", and so must the case be with "certain". Is there a specific grammatical-linguistic term for these "either is or not" adjectives like "dead"? @ Amanda, yeah; so thought I but then, certainty is [per definition?] "100% probability". I guess my premise is false.