I do believe it should say to not exceed 2 hours, rather than run less than 2 hours. (Although some do exceed 2 hours, like Die Hard 4. But they made that PG-13, so it doesn't make it alright).
If you simply don't understand:
Say a movie theatre is given 10 hours to a certain film. A movie that runs for 2 hours 15 minutes shall only be able to run 4 times with 1 hour to spare, which means only making money from 4 showings. A 2 hour movie will be able to run 5 times, making money from 5 showings.
Unfortunately Fox will haven't grasped the fact that a movie filmed to run for 2 hours can be good, but a movie filmed to run for 2 hours 15 minutes being cut to 2 hours will not.