This is really about the power of intonation; punctuation comes into it only insofar as it can be recruited to convey the intonation of these two utterances. The first has the intonation of a declarative, the second the intonation of a type of interrogative (with a final rise) — in fact, a type of reclamatory question, used to seek a repetition of something you haven’t understood or an explanation of something you understood but can’t accept.
The exclamation point (!) is used to indicate emphasis (associated with greater intensity and/or higher pitch), the interrobang (?!) to indicate a combination of interrogativity (rising final) and emphasis.