Discrepancy between total number of reviews of deck and collection in Anki stats

In the stats window, I checked the total number of (past) reviews for the deck and for the collection. The number of reviews for the collection was higher as compared to that for the deck. But, since they contain the same cards, the number of reviews should be the same in my opinion. What is the reason for this discrepancy?

‘Collection’ includes review history from deleted cards.

But, why shouldn’t the review history of the card be deleted when the card is deleted?

One problem I see with the current approach is that if a user decides to delete most/all of their existing cards and then start creating new cards, the collection would always have the burden of all the old data without any apparent benefits. In other words, the collection takes up more space on the hard disk and more space on AnkiWeb without any apparent benefits.

That data needs to be retained in order for the graphs to accurately reflect the total amount of time you spent studying in the past. Some people place great importance on maintaining their streak, and would not want previous review counts to drop (perhaps to zero, breaking the streak). And if some future algorithm comes around that uses the full sum of your review history to tailor scheduling to you, the more review history that’s available, the more likely it is to be accurate.

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