Must have = probably, likely, supposedly, It is used for situations when you make a guess. Example you invite a friend to visit, the friend lives 3 hours driving time from you, you know they left to come to you 3.5 hours ago. You or anyone else would or could say he/she/they/persons name 'must have' arrived by now. You use must have whenever it is possible to do some calculation or reasoning. Or sometimes supposition.

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Both Steve and John omit the other possible meaning of "must have", which can also be used if you believe that, in spite of some small uncertainty, there's strong evidence that someone did it--so they "must have done it." For example: Dick: I think Sally must have eaten the last chocolate pudding from the fridge, don't you, Jane? Jane: She must have eaten it--just look at her mouth and hands! If you wish to indicate uncertainty you must use a questioning tone of voice whereas a level or emphatic tone, as when stating a fact, indicates you're quite sure (based on the evidence).

"must have" indicates uncertainty.....a supposition.....one is guessing what happened. "will have" indicates certainty (it's a fact)....as one is telling in a true story