FWIW, this is what Fable said
There is a benign filter that explains the data without rejection-sampling-for-easiness: the grader needs its own Monte-Carlo ground truth per MLP, and high-variance MLPs are exactly the ones whose MC truth is expensive or uncertifiable. If the organizers kept only truth-certifiable MLPs, that filter truncates the variance tail — and it would apply to the private 50 as well, since manual validation needs reliable truth even more. So the private set may look like the LB 50, not like the full split.