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I think it truly depends on the situation and the context. Sometimes people know that a person is not right for them and that's the nicest, albeit cliched way of saying it. Other times they could be hiding something and don't know how else to say they no longer want to be with you.
Humans are constantly evolving and changing, and sometimes I think are needs and wants change and it's no fault of the other person, so in that case saying "it's not you, it's me" would be entirely accurate.