The provided map says 1 square = 5 feet. Yet the description for N13 is as follows:
The roar of churning water fills this massive chamber, which is some seventy feet across. The floor drops away for most of the length into a vast pit of surging seawater, amid which rise several stony pillars. The sides of these pillars are jagged,but the tops are relatively flat, though they’re covered with strips of seaweed and seething with crustaceans while being constantly splashed by the waves below.
This large room is empty, save for six iron-framed, ten-foot wide mirrors affixed firmly to the floor.
The creature is pushed back 20 feet...
Emphasis mine. With three definitive examples, I'm fairly certain it's the map scaling that off, and should be 1 square = 10 feet, but just wanted to bring it to your attention and get some confirmation.