premise: How Green Was My Valley is a BBC Television serial based on the novel by Richard Llewellyn, and features one of the last performances by Stanley Baker. It was first shown in the UK from 29 December 1975 in six weekly parts, while producer Martin Lisemore also cast Siân Phillips in his next production, "I Claudius" (1976).
hypothesis: "I Claudius" was released to the public over ten years ago.
reason: Technically, 1976 was over ten years ago. I think the system got this wrong because it was an arbitrarily selected period of time.