Do they always play the negative before the positive? If so, maybe people are just getting better at the game as they play more.
I got 13 points after the negative image (first), and 11 points after the positive image, which contradicts the research. I was in a pretty positive mood before starting though, and then I started concentrating on the task, which may or may not have had an influence. Also, the "positive/humorous" video I found to be pretty boring, so maybe I got more annoyed than getting in a good mood, because I just wanted to get on with the test.