I sometimes manage to get that to coax a broken link to e.g. threepanelsoul, which at least gives me the title to try and further search for.

Sometimes.

Would have bookmarked your link :) except that it doesn't seem to index xkcd comics.

I tried "xkcd duty call" and it gave no results [0] while Google lists the comic as the first result for the same keywords.

[0] http://ohnorobot.com/index.php?s=xkcd+duty+call&Search=Searc...

I'd say XKCD is a special case. Considering the "There is always a relevant XKCD" meme, I think google has a pretty decent grasp of what terms relate to what XKCD comic. Not to mention the transcripts and explanations of explainxkcd.com.

Another possible explanation is that Google's search engine is exhibiting, due to the lack of a better term, a form of "transference" [0].

Google engineers are huge fans of XKCD as is evident from Randall Monroe's well attended talk at Google some time back. In my mind, his talk was easily one of the most attended talks, second only to Linus Torvalds talk on Git.

[0] I'm sure there is a standard term for software exhibiting biases held by its authors when making decisions on behalf of users.