dx dy and dz is broken

When entering the command: /testfor @a[x=30,y=4,z=41,dx=39,dy=13,dz=50] it's fine, but when I enter that area, it works, but the box is way bigger than it should, and I don't think it's because I entered it in wrong. This website on my phone is not working well at all and is pasting random stuff from my phone btw.