Okay, I understand it's now known mainly as the Ryman League but I can't find out why it was ever known as the Isthmian League originally.

I also find this table quite interesting:
http://en.wikipedia.org/wiki/English_football_league_system#Structure
It shows us that for some reason the Bristol Downs Football League goes lower than anyone else.
http://en.wikipedia.org/wiki/Bristol_Downs_Football_League

Also, the North/South divide thing seems to imply that if too many teams from one end of the country get pushed down they get moved from North to South or vice versa in terms of league. That sounds a bit harsh and weird.