jack

Part of it is that in the first case you're describing a social role, and in the last you're referring to a physical characteristic, and one that people are often prejudiced about. People would likely think it odd if you said "I saw two tall people walking across the quad, and one of them said 'and, ninethly'" because their being tall feels less relevant than their being professors. ("If it was "I saw two tall people, and one of them said "I've always hated basketball," there would be a connection to their being tall.) Again, if you said "I was in the grocery store, and one of the other shoppers said..." or "someone carrying a stack of pies said..." there'd be an expectation of connection. I could come up with examples where the man in the grocery being black is relevant, of course, but usually it's not.