"The sound and the fury" and "As I lay dying"

http://www.amazon.com/The-Sound-Fury-Lay-Dying/dp/B00005WWS8

To me it's quite odd to publish "The sound and the fury" and "As I lay dying" together as 1 book.
Why would anyone like to do that? Because these 2 novels were written by William Faulkner? Because they are set in the same place? Because they are about families? Because they both have multiple narrators?
Those are superficialities. 
Like "The sound and the fury", which deals with the Compson family (Mr Compson- Jason III, Mrs Caroline Compson and the children Quentin, Caddy, Jason and Benjy), "As I lay dying" is about the Bundren family (Mr Anse Bundren, Mrs Addie Bundren and the children Cash, Darl, Jewel, Dewey Dell and Vardaman)*, but the central character of "The sound and the fury", as I've written, is Caddy and each of the 3 1st sections is a response to her, to her sexual promiscuity and to the impact she has had on the narrator (and the 4th section, one may argue, is about the void and meaninglessness in the Compson household after she's gone); whereas the central character of "As I lay dying" is the mother, Addie Bundren, and the book is a record of people's reactions to, thoughts on and feelings about her and her death. This leads to the fundamental difference that "The sound and the fury" uses the Compson family as a setting, a means through which Faulkner describes the changing South in America, the conflict of values and lifestyles (most clearly demonstrated by the conflict in Quentin's mind), while "As I lay dying" deals with the family and explores the relationships in it- between Mr and Mrs Bundren, between Mrs Bundren and each of her children, between the children. 
Obviously, "The sound and the fury" is also a bit concerned with family relationships (Benjy's love and longing for Caddy, Jason's spite at his siblings, Mrs Compson's preference for 1 son, etc) and the family is not the only theme in "As I lay dying", it's also about mortality and the question of existence. But when one looks at the core of each book, and compares, one can see that these 2 books are fundamentally different, with different concerns, topics, themes and tones. Which is why I find it odd to put them together (as much as when anybody compares "The catcher in the rye" to "Naive. Super." by Erlend Loe and that book by John Green- either "Paper towns" or "Looking for Alaska" **- "The catcher in the rye" is closer to Sylvia Plath's "The bell jar").




*: 1 similarity I notice is the mother. Addie Bundren loves only 1 of her children, Jewel, because she hates her husband Anse and Jewel is not his. Caroline Compson also loves 1 of her children, Jason, for a different reason, that to her Jason is the true Bascomb whereas all the others are Compsons. This has attracted my attention, and intrigued me, right from the beginning and I found it very tempting to see them as the same, which I've still been considering, but now I suppose they're different in certain ways. I may come back to this later.
**: both of which I have read, 1 of which is compared to "The catcher in the rye" and none of which create a very strong impression on me, therefore I neither remember which one is compared, nor remember what they actually are about, apart from the certainty that I've read both.