When you say that UFC is unranked, I presume you're referring to the AP ranking. The more I watch the AP ranking the last impressed I am, but my views are obviously not relevant. The more relevant point is the AP ranking officially is not even a consideration for the selection committee. They have a huge pile of data, the AP rankings aren't among them.
They do have NET rankings where USC is 20. That's not 16 but it's on the outside looking in and things need to go right which they probably won't, but 45 is the approximate cut off for being invited to the tournament and it's my observation the 20 is a lot closer to 16 that it is to 46.
Quad 1 wins appear to be important. Here is the number of quad one wins by the current bracketology 4 seeds:
Duke 3
Ole Miss 2
Oklahoma 2
Tennessee 4
USC has 3. Not as many as Tennessee, exactly the same as Duke and more than Mississippi or Oklahoma. I'm not arguing that USC deserves a four seed over any of those four, I'm simply arguing it's not is ludicrous as some people seem to think.