Interesting... It is a bit confusing since the rules say "Approaches can be either narrow (as wide as one piece) or wide (as wide as two pieces)."
That makes it sound a little bit like the width (narrow or wide) determines if one or two pieces can be on that approach.
I.e., the physical space on the map seems to be designed to hold 1 or 2 pieces - but in fact that physical/graphic mnemonic is only to indicate the number of "leading" pieces in combat.
I believe the map was designed so that areas of "like" terrain characteristics were abstracted into locales during the map's design.
Bowen probably saw that he needed a way to address odd-length sides, chose to redraw the areas as needed, then define them as single- and double-width lengths intentionally.
The terms "narrow" and "wide" were likely coined to describe these different-length sides, not to alter a locale's capacity but to address the fact that some locales may have longer sides.
More than likely this also led to the rules addressing attacks crossing wide approaches.
Mind, all this is a guess on my part!