dx, dy, and dz go out one block further when xyz are set to ~~~

As you can see in the attached video, if you run a command that has dx dy dz and also uses xyz as ~~~, the area you select will go out one block further then intended (notice the comparator activating even when I'm not in the area specified by the command block). (also shown in the video halfway through) If you use actual numbers instead of ~~~, then the issue doesn't happen.