Created attachment 91958 [details]
Comparison of DejaVu Sans Mono, Droid Sans Mono & FreeSerif dashes.
For some reason, in DejaVu Sans Mono, the em dash (U+2014) and the en dash (U+2014) have the same width -- the maximum character width of DejaVu. They are also not as wide as the "m" or "n" characters, respectively.
Other monospace fonts solve this better, such as e.g. Source Code Pro or Droid Sans Mono. (The included FreeSerif comparison is a bit of an accident, but maybe shows how a "normal" behaves.)