One thing I thought of a few years back. Ruth Berman's notes on Greg Jein's "Jonathan Doe Starship" talks about several different roles, with ships that are based on the same hull but have different internals to do different things. There are explorers, deep space explorers...I think scouts were mentioned. I always thought this would explain the wide range of numbers for ships which look the same, as well as the Archenar class. It wouldn't explain Jeffries' numbering plan, unless his "desgn" is the same thing as Berman's "role".

FJ's lists assumed a more or less sequential order of registry and class, with the 18xx numbers being intended for variants of the base Constitution model (with the USS Constitution being 1700). IIRC, it's mainly the Tikopai variant being the 1800 class, with SotSF implying that the class didn't fully enter service until the movie era due to various delays, and then with some of the original batch ships never built at all.