 Notice that the first sentence doesn’t introduce uniqueness, so the pronoun introduces something that indefinites do not. What this means is that whatever account we give of pronouns—through dynamic semantics or whatever—we will want to account for this variation in interpretation in a principled way. We want to know precisely why a pronoun looks like a definite description here, but an indefinite description there. Whether extant accounts are satisfactory in this respect is subject to debate.