Dunno if this is the case for wherever you got your sales figures from, but a lot of the places that track best-selling games only track physical releases. Or they might also track digital releases if the publisher provides them to whoever is doing that tracking, but they often don't. BG3 does not have a physical release (yet).
And a quick google seems to back that up. According to Phil Spencer the other day, Starfield has had "over 12 million players". I'm assuming this is a combined figure of sales and people who downloaded it through game pass. So, less than 12 million copies sold, and probably a good deal under that cuz I assume game pass would be a pretty decent chunk of those players reached. If the top result when googling is accurate, BG3 has sold 22+ million copies. Prolly enough to crack that top 20, I'd guess.