For a possibility-based comparison, the claim about the subject is in question, whereas the analogous feature in the comparison term must be sufficiently accepted. If both instances are equally contested, the comparison does little to remove the initial objection.
A person might argue that a pupil from an ordinary school can be exceptional, just as fruit from one garden can differ greatly in quality. The garden analogy makes the possibility intelligible; it does not establish, without evidence, that the particular student actually achieved excellence.
Thus showing possibility differs from proving occurrence. The former removes the claim of impossibility, while the latter establishes a fact. The rhetorical purpose under discussion concerns the former.