I've been wondering about this both from watching the pros and from my own running. It seems some runners have the "wrong" body type for their events. Taoufik Makhloufi looks more like a 400 guy than an 8/15 type, Bolt looks like a 400m guy (and used to be one), Asbel Kiprop looks like a 10k+ runner but is really an 8/15 type...there are many examples. They obviously have the right muscle fibers for their events, but their builds don't match most of their competitors.
In my case, I thought I would be a 3200m runner when I came out for track my frosh year of HS because I have the distance runner build (tall and thin, think Kiprop). But it turned out I had quite a bit more speed than I thought I did, and now a year later I'm pretty sure (but not certain of course) that I'm pure 800m or possible 400/800. I'm always getting dropped on tempos and long runs as my muscles start to tighten up, but I can outrun faster teammates if we have to hammer a couple 200s or something like that.
How often do these "anomalies" happen? Like a short guy with huge muscles turning out to be slow-twitch dominant, or a tall skinny runner actually having the muscle fibers of a sprinter?