My thought is that the C ratings mean just what the definitions say that they mean. C1 means brand new and not previously occupied. etc. etc. C2 means not new but renovated to like new condition. etc. etc. In most markets, if the ratings are applied properly, the vast majority of homes will be either C3 or C4.
The problem with using "rough correlations" is just that, they are rough correlations. For example, a home renovated to like new condition so that it is in "Excellent" condition, is still C2, not C1.

And, a home might be in good or average overall condition, but still be a C6 due to one significant issue.
Read the definitions and apply the ratings according to the actual definitions, not according to some roughly corresponding scale that someone else invented.