Well, it new, innit? In other news Gone with The Wind isn't #1 at the box office right now.
True, it's not a surprise that Theros is leading new releases, but that it jumped two spots on the D&D best seller list is different. Isn't that based on total sales of a book?
If I'm right it means Theros' sales have already out paced Ravnica and Saltmarsh's sales before it's been released. Of course Wildemount blew past everything basically but the core and starter set.