But that's the point--Christie's novels (and even more her short stories) are not so much about crime or vice or some sort of universal themis but epistemology--the limits of human understanding and the reasons we place blinders on our ability to see the truth (except for Miss Marple and Hercule Poirot--but even they have their "why didn't I see that?" moments, hinting at a cave lector warning). Christie's novels aren't puzzles but explorations of human limitations. Overall though (compared to the works of other writers, except maybe Ellery Queen and possibly John Dickson Carr), her novels as fair. as they can be, given her overall theme.

I think I've suggested this to you before, but check out "Towards Zero" as her ultimate statement about indeterminacy and deception. It's the ultimate challenge to the reader of mysteries, even more than Roger Ackroyd..