missing the (inside) joke

I have a new coworker named Mark, who has lived in the Bay Area for nearly ten years but still has a thick Australian accent. That makes two guys named Mark in my group, and to distinguish between them, we call one "intern Mark" (he was an intern a few years ago before joining full-time) and one "Maah-k", which is an exaggerated version of what it sounds like when he says his own name. I don't think he's entirely amused by it, but it's kind of stuck.

Anyway, D met him for the first time last week, when we went to dinner with a bunch of my coworkers (as an aside, Choi's Kitchen, in Santa Clara, is delicious). The conversation went something like this:

D:	Hi, I'm D****.
Mark:	Hi, I'm "Maah-k".
D:	Hi "Maah-k".
Mark:	(a little irritated) Yes, I get it, I have a funny accent!
D:	(confused) Huh?

In his defense, D honestly had no idea why Mark was so touchy about the pronounciation of his name.