It wouldn't surprise me at ALL if Fox optioned for a PG-13 release for the theatrical and then an unrated for the dvd/blu-ray. From a business stance they could make all kinds of money. Do I agree with it? No, but I wouldn't put it pass them is all I'm saying. But hopefully, RR will have some say in regards to the rating.
And also, the PG-13 rating for AvP was the least of that films problems. It was just a bad film. End of.