Shabbos 37 – שבת לז

The Gemara explains that Rav Sheshes needed to teach us the diyuk (inference) in the Mishna that bread which crusted is permitted.  Rashi adds that this diyuk was not self-evident.  However the Mishna seems to state clearly that the issur is merely if it won’t crust before Shabbos, which clearly implies that crusting is the benchmark.  Why is this not self-evident?