Originally Posted by
Aldoman
The sounds that ん makes in combination with the syllable after in some situation is very natural to adapt and imitate for some speakers (spanish-natives like me) that I had never realized the small variations that sometimes made it sound like an m or the other variations (for example, the way we move our lips to pronounce sanpo does make me pronounce it as sampo). I hadn't even realized that they romanize 新橋 as shimbashi! I guess you learn something everyday.
Further reading:
http://en.wikipedia.org/wiki/Hepburn_romanization
Spelling things as "-mp-" is traditional Hepburn (Nihommatsu, Gumma, Asahi Shimbun, etc), while "-np-" is revised Hepburn (Nihonmatsu, Gunma, Asahi Shinbun).