Computer generated voice has certain artifacts that makes it a bit distinctive. Auto-tuned natural voice has similar artifacts, so if an artist uses auto-tune, then there's less noticeable difference between their real (computer-altered) voice and a fake computer-generated voice.

A more exaggerated example would be artists that use a vocoder (e.g. Daft Punk or something like that), which would be even easier to synthesise convincingly because the real vocals are also mostly synthesised.