I explained in a post a while back how we're computing "c" as an average based on multiple codon messages for one amino acid string, rather than a simple true count based on just one codon message for one amino acid string.
It's not important right now to recount the details of this "c- average" computation, but if and when it's appropriate, I think I should recount it so that you can decide whether "c-average" actually does contain measurement error.
I'm pretty sure it does, because I could have selected a different set of messages for each amino acid string, had more been available in the public databases.