Has anyone come up with an answer for the question that you posed mid-text? The question about the one base change that did _not_ lead to an additional C or G

Has anyone come up with an answer for the question that you posed mid-text? The question about the one base change that did _not_ lead to an additional C or G, the CCA -> CCU modification? (Note to myself: I think the question is: Why CCA -> CCU and not CCA -> CCG)