I don't remember an official description, but my guess is that the dupe detector is hard for a few hour and detects any reposts, and then go to a soft mode that only detect dupes with a lot of upvotes and/or comments in the last year or so.
So if some post gets a lot of traction or gets to the second chance pool after the other has been posted, there are two live posts. This is fine after a few days, but for a few hours it's a tiny problem.
Probably dang/tomhow change the details from time to time without warning, but this is guess model matches the behavior I saw.
