Cuboid Target Selector Not Functioning as Intended

Cuboid Target Selector does not appear to be working properly.

For example, I can get this to work:
/kill @a[x=-1208,y=2,z=410,r=1]

but not this:
/kill @a[x=-1208,y=2,z=410,dx=-1177,dy=2,dz=441]

or this:
/kill @a[x=-1208,y=2,z=410,dx=~31,dy=~2,dz=~31]

or even this:
/kill @a[x=~10,y=~0,z=~-20,r=1]

However, Relative and Absolute coordinates work just fine when not being used in target selectors.

For example:
/execute @a ~ ~-1 ~ testforblock ~ ~ ~ minecraft:coal_block