I have a feeling that if it was just 2 days, it wouldn't swing the difference so dramatically compared to PS/PC. It might be that I'm thinking about player behavior when I should be thinking that the stat might not be counted the same between platforms?
Maybe xbox count total keys sold to how many finished the game, while other count the total as # of players that started the software once? I tried a quick search to see if that info is publicly known but seems to be a conclusive ¯\_(ツ)_/¯
I've found several posts stating that steam tracks owned vs achievement ownership, others that its people that booted at least once vs achievement ownership, so I really am not sure. If its ownership, 20% is a crazy high rate though. I've seen what people's backlogs look like after a few sales.