You think too much in balance and gamey terms.
That is not how the worlds of BM were built.
The original numbers (which have since been changed many times, though) were created in a painstaken process of dedicated research into medieval history, economics and details of farming and trade, which culminated in an hour or two during which I sat down with the database in one window and the map in another and looked at the regions and simply made up numbers for them that kind of felt right.
No, seriously. There was no research or any balancing or calculations of bla, whatever. I looked at the regions and said "hm, ok this looks like fertile farmland, let's give it a good food score".
Because, let's face it, that is largely how the real world works. There are so many factors determining fertility of an area that from a higher level, it looks pretty much like someone pulled it out of a hat.
