Thats true. The boundaries are porous though, Rushden were in the Northern league south last season. I have had a quick look on wikipedia at the Isthmian league, luckily it has a map of where the clubs are situated geographically. If we put Moneyfields in the Isthmian league south, then one of those clubs need to move to the north division.
Barking and Thamesmead town are in central London, and by the wiki map, are a stones throw apart. Barking are in the north division, Thamesmead in the south. If the FA moved Thamesmead to the north it creates a vacancy for Moneyfields. It also creates a local derby. With Thamesmead in the north, a north club can move to the southern league central. Soham or Mildenhall can the move to the southern central in place of Moneyfields.
I have also had a look at the mileage on google for the Rushden v Moneyfields game, its 135. This would be the case for the Bedfordshire clubs, and i cant think of an easy route. It cuts both ways, cambs and beds clubs have to do that journey in the opposite direction. I havent looked at the geography of the central clubs, i know Beaconsfield are on the orbital London road.
I wonder how many anomalies like that there are. What i do know is that you cant argue with a dictatorship like the FA, and you must do as your told. One note of interest. Thamesmead have a ground capacity of 6,000, according to wiki, thats huge by our standards.
Intuition tells me you will disagree with this assessment, and i would be grateful if you can prove me wrong. After all i spent 15 minutes on this project, if you can call it that.