I'd bet this is due to a "smart typography" processor that converts runs of hyphens to em- or en-dashes. The standard typing convention for decades has been to

I'd bet this is due to a "smart typography" processor that converts runs of hyphens to em- or en-dashes. The standard typing convention for decades has been to use "--" to indicate an em-dash, and that's certainly what I've learned. Many smart converters (including SmartyPants, the one John Gruber created as a companion to Markdown) make that conversion. However, there's also the TeX convention of converting "--" to an en-dash and requiring "---" for an em-dash; some processors will use that standard by default, like Pandoc and MultiMarkdown. If you're typing "--" for em-dash but you're unknowingly using a processor that turns that into en-dashes, well, you get this. (At some point, Tumblr converted their internal Markdown processor from "--" for em-dash to "--" for en-dash, and hundreds of my tech blog posts now look stupid because all the em-dashes have become em-dashes. Thanks, Tumblr.)