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FindingPopularItems and MostWatchedItems are 2 different algos serving 2 different purposes.
The key difference between the two algos is one uses keywords, and the other uses (purely) watch count. Watch count limits us to only unique items, so naturally it is very limiting. Also, watch count typically recall items that are rare, which means low churn. Popular keywords on the other hand better reflect what the mass shoppers want, which help us churn our inventory much faster. Naturally these two algorithms serve very different purposes. Watch-driven popular item acts as "water coolers" to get people to talk about things on eBay, while keywords are meant for search.