Comment by phlakaton

I don't think it's so much a matter as en-dash breaking the rules and em-dash following them. Robert Bringhurst argues, in Elements of Typographic Style, that em-dash is one of several Victorian innovations that he feels impede the flow of text, and en-dash (with spaces) is a more modern, typographically balanced alternative.

I conclude there are therefore two different rules you can choose from (as in many points of style) and I – a hacker, artist, and typographic enthusiast – picked the one I like best.

Though I revert to em-dash when parodying Emily Dickinson—as one should—