Having grown up on the edge of your main discrepancy zone, I can add context for the non-Brits.
Large parts of East Anglia (the Fens) are effectively reclaimed land having been swamp until being drained in 18th century. The city of Ely is still known as the "Isle of Ely" having once been effectively an island, and some of the rivers around there flow a good 10 feet above the surrounding land.
