The problem is, that the consensus about a particular developer in a particular project -- to call that developer "10x, 1x 0.1x" -- is a consensus of peers and

The problem is, that the consensus about a particular developer in a particular project -- to call that developer "10x, 1x 0.1x" -- is a consensus of peers and management. And peers almost always lack the information to make the objective call. If they'd had that information, they wouldn't have called that developer a 10x. And that's one of the reasons, why you never see normal people calling themselves 10x, by the way.