Why is it called 'The Devil's Arithmetic'?

-1+1=0

I AM DINOSAUR...RAR!!!

The short answer? Because that's the title of the book it's based on.

There are a couple reasons given in the book. Hannah refers to one of them while she's in the camp: I am alive one day, I am alive another day, one day plus one day equals two days that I've been alive. She refers to this as the Devil's arithmetic. Another answer given was in the author's note in the back (which I realize may not be in every edition). Yolen (the book's author) talks about how many people died in each camp: 200,000 at Sobibor, 1.1 million at Auschwitz, 700,000 at Treblinka, and calls those numbers the Devil's arithmetic. A third reason may be because of the ID tattoos. In the book (don't remember if it's in the movie), Rifka and Hannah talk about the individual digits of their numbers, saying what each one signifies to them. (1 is for Hannah, 4 is for her family, 7 is for Rifka's family, and so on)

In the movie, Hannah's grandfather(?) talks about the Nazi's adding and subtracting Jews until there were no more left.

Any of the above reasons I think could be interpreted as the right one.