"1,000 Most Common" collection size has grown beyond 1000

I’ve been playing the “1,000 Most Common” Italian collection for a few months, just doing a quick 10 questions a day as maintenance, so I have only “mastered” 79 sentences but I was “playing” 990 sentences. I was therefore curious what would happen once I reached 1000 sentences in play, with still so few of them mastered, because on one hand, every question beyond 1000 would have to be a review question, but on the other hand, I have the “Max reviews per collection per day” set at 20.

So I continued playing to 1,000, and am very confused by what happened: the “1,000 Most Common” collection has now ballooned to 4,007 sentences, with 1,006 (25.106%) played. What’s going on here, and what is the source of the extra questions - has the collection just duplicated itself?

This question has been asked before:

Thanks for the link. What baffled me was that before I got to 1000, it definitely showed I was at 990/1000; the target only went up once I hit 1000, but in hindsight I guess this is because I had “progress chunks” turned on.