What does "one thing" refer to here? Why it is not included in the subtitle?
Great question. When Ross first arrives he has a lot of bags and Phoebe jokes that he thinks the barbecue will last a long time and that's why he carried that luggage. But of course, we realize Ross is going to China. Because he immediately responded to Phoebe's joke by saying he's going to China, she says "you say one thing" and then makes a sound which is like a low scream. Here the "one thing" she is referring to was the joke about him going to China. She is acting as though him going to China is a direct reaction to her joke, even though he's going on a work trip, which is itself a joke. The conversation between Ross and Phoebe is basically just a long joke. The phrase 'you say one thing" is actually often said in response to when someone reacts dramatically to something you have said. In this case, it would have been very dramatic for Ross to move to China because of Phoebe's joke/comment about his luggage. Ex: A: That dress looks a little tight. B: That's it. I'm going on a diet! A: Really?! You say one thing! Ex: A: I don't think that looks right. B: Fine! Do it yourself! A: Geez, you say one thing.