Well, the "cleanliness" is probably because it hasn't been open-source for too long - most of the original work was done by a dedicated team of software engineers most likely operating under fairly strict code guidelines.
As someone who works on a project with a team of dedicated engineers under fairly strict code guidelines including code reviews, I can tell you from experience none of these factors have to do with "code cleanliness" (as ours is not, at least in my opinion).