the degree of certainty "must have" -- bigger "may have" -- smaller

It's much easier to explain if you give a sentence. It's not very helpful to give fragments, especially for modals. In some contexts, "must" and "may" are used for deductions. Using "must" means you're very sure about the situation, and "may" means you're thinking it's possible, but you are not fully sure. It's not necessary for me to explain the "have" part - I think that will confuse you more. Look at this: https://learnenglish.britishcouncil.org/en/grammar-reference/modals-deduction-present

I think "must have" express the prediction and this prediction rate is nearly 80%, and "may have" express the possibility . I think so

Must have implies a need, or necessity whereas "may have" suggests a possibility.