Indeed the cause of the high id for authors is historical (converted data).
Ever since installing the first Moku version of Product Type Book, I've been modifying and adding also quite a lot, so upgrading to version 1.3.8 will be not a straight forward job, also the bookstore does not see the advantage of having the latest version, they are OK with my "tailor made" version.
I'll dig into the several versions to see if I can find out what is causing this weird problem.
Thanks a lot for your explanation and all the links, this makes my job easier.