Certainly it is fascinating question to think about why any novel was given the name that it bears. There are two places in the novel that refer directly to the "devil's arithmetic" which explore why this concept is so important to the novel as a whole. Firstly, at the beginning of Chapter 16, Hannah/Chaya comments on her life in camp, talking about how each day that she remains alive is a cause for triumph:
Part of her revolted against the insanity of the rules. Part of her was grateful. In a world of chaos, any guidelines helped. And she knew that each day she remained alive, she remained alive. One plus one plus one. The Devil's arithmetic, Gitl called it.
Note how the title here is used to reinforce the desperate battle for survival that the Jews were engaged in, where every single day that they survived was a battle won and success.
In Chapter 17, the devil's arithmetic is given a different meaning as Hannah/Chaya desperately awaits for news of the escape plan from Gitl:
The days' routines were as before, the only change being the constant redness of the sky as train-loads of nameless zugangi were shipped along the rails of death. Still the camp seemed curiously lightened because of it, as if everyone knew that as long as others were processed, they would not be. A simple bit of mathematics, like subtraction, where one taken away from the top line becomes one added on to the bottom. The Devil's arithmetic.
Note here how the survival this phrase refers to explores how the deaths of the nameless Jews means life for the Jews working in the camp. It seems harsh that the Jews in the camp are able to be "lighthearted" for such a reason, and yet we must remember that the holocaust was an exceptional situation where the desperate struggle for survival changed the order of ethics and morals.