This is by design and explained on the blog in the episode entitled, "Three Markdown Gotchas"
1) Markdown’s single biggest flaw is its intra-word emphasis.
I don’t think anybody writes:
often enough to justify making it nearly impossible to talk about tokens with underscores in them:
is interpreted as:
It even works across word boundaries:
file_one and file_two
file<em>one and file</em> two
Whenever you’re writing tokens with underscores you have to make absolutely sure you’re in a backtick-delimited code span. The same problem will also nail you on equations like a*b*c, but that seems to pop up less frequently.
Showdown follows the reference implementation on all this, but in WMD I do a little preprocessing to hack the idiocy away: basically I just backslash-escape any underscores or asterisks that might trigger it. It’s a flagrant violation of the standard, but since it’s a pre-pass that should produce identical output with any Markdown processor, I feel justified. Unfortunately my hack did screw up one edge case (which I don’t have in front of me) and there isn’t any way to disable it. Both those things will change in the next release.
Basically, if you want to have italics or bold markup in the middle of your words, you're going to have to whip out the HTML tags and tag them up that style.
Basically, if you want to have i<i>tal</i>ics or bold markup
in the mi<b>dd</b>le of your words, you're going to have to
whip out the HTML tags and tag them up that style.