I don't think that's the issue; if you look at the graphs, the standard deviation is tiny compared to the variability between stories, and in some the spoilers/no-spoilers don't even overlap. The stats:

For all three experiments, analyses of variance revealed a significant effect of condition. (In order to control for variability between stories, we analyzed the data by comparing different versions of the same story.) Subjects significantly preferred spoiled over unspoiled stories in the case of both the ironic- twist stories (6.20 vs. 5.79), p = .013, Cohen’s d = 0.18, and the mysteries (7.29 vs. 6.60), p = .001, d = 0.34. The evocative stories were appreciated less overall, likely because of their more expressly literary aims, but subjects again significantly preferred spoiled over unspoiled versions (5.50 vs. 5.03), p = .019, d = 0.22. In all three story types, incorporating spoiler texts into stories had no effect on how much they were liked, ps > .4. Subjects also did not indicate in their free responses that they found these altered beginnings out of place or jarring.