Killer break-up line #3: "It's not you, it's me"

This break-up line is a mystery to me, I admit. What exactly is the person saying? Whoever says this line is assuming responsibility and/or blame for the break-up, that much is clear. But beyond that, it's as clear as mud. Once again, it's supposed to spare the dumped the misery of being dumped. But does it, really?

It's a killer line because: It doesn't really tell you anything. It doesn't explain anything. Most of the time, people who use this line don't even want to give you a reason or an explanation. Thus, the extremely vague "it's not you, it's me".