When you want to execute a command with in a cubic area, the selector do not use the "dx", "dy" and "dz" value.

Step to reproduce:
Make sure you're not in (0,64,0)
Type "/tp @a[x=0,y=64,z=0,dx=1,dy=1,dz=1] ~ ~10 ~"

The command is still executed even if you're not in the area of detection