Note, though, that even if the statistical analysis is correct (which it might be), this still doesn't tell us why this effect is present (as the paper makes clear). One possible reason, which the paper points to, is the conventional wisdom that Yale students learn a lot of theory but not a lot of doctrine.
But there are other possible reasons. For instance, perhaps Yale has such an excellent reputation that Yalies who are relatively far down in the class get more clerkships than comparably ranked students at comparable quality schools. (I have indeed heard that this is the case, though I have no hard data on the subject except for this appellate clerk data.)
Say, for instance, that the Yale district court clerks generally come from the 25th to 75th percentile at Yale, and on average represent the 50th percentile. But say the Harvard district court clerks on average represent the 55th to 85th percentile at Harvard, and on average represent the 70th percentile. And say the Yale and Harvard student bodies are on average comparably good (or perhaps even that Yale is on average better, but only by a little), so that the people at the 50th percentile at Yale are a little less good at legal analysis than the people at the 70th percentile at Harvard.
The higher reversal rate associated with Yale clerks may then just reflect the lower average quality of Yale clerks, not the lower average quality of Yale graduates or of a Yale education. And this lower quality would flow from the school's reputation exceeding its actual merits (even if its merits are very great).
Likewise, the paper itself points to some other possible explanations, for instance "a grading system that is not sufficiently partitioned to allow judges to identify the quality of applicants": "Officially, there's a system of honors, pass, low pass and fail, but three-quarters or more of the class gets a pass, and professors rarely give out low passes ...." That too would be a way in which the average quality of Yale clerks might be lower than the average quality of other clerks (if it is), even though the average quality of the entire Yale class might be as good or better as that of other schools' classes.