You're perfectly right and it's what occurred to me when I lived in Japan, too: you do get to see 'abberant' spellings quite often and that's because train companies decided many, many decades ago on what system to employ for their English station names, and those stuck of course because of printed schedules and so on. Depending on how you count Nihombashi either gets lots of points b/c of the many people who get to see this particular name each day, or it just gets a single point because it's one spot on the map. Observe that when you click through the pictures to get to the actual map—you'll find "Nihonbashi Bridge" spelled in the more common way (including the adorable repetition). So arguably 'Nihonbashi' it is in general but the station name is spelled with an 'm'.