(Note that I know that dx, dy and dz are DIFFERENCES in x, y and z)

Selecting a box area using dx, dy and dz doesn't work.
when executing:

 /say @a[100,55,100,0,dx=10,dy=10,dz=10] 

it will not work at all.

try using

 /say @a[100,55,100,3,dx=10,dy=10,dz=10] 

confirms that it is a bug with dx,dy,dz, because getting in a 3 block range of 100,55,100 will trigger the command!

more details and "proof": https://www.youtube.com/watch?v=7vsmoJ0xXbQ