I’ve heard this from a few people, that Mitt Romney is the “moderate” GOP primary candidate. It’s not based on his positions, obviously, but I also wouldn’t assign it to his flip-flopping.
It probably has more to do with the way he talks. He’s not visibly angry at the world and he doesn’t talk about his church and God every other sentence. He just seems less extreme than Gingrich or Santorum did, and since few people in the media actually care about the policies he supports people don’t know how to weigh those.