This video demonstrates a strange bug in the way dx,dy,dz are interpreted in selectors:

https://www.youtube.com/watch?v=RuKv11Ivvc0

Sometimes the selector area is exactly 1x1x1 and sometimes it is slightly larger. It seems this can be decided by previously run commands.

This is exactly what is happening in the video:
1. The first device runs a testfor command on a clock and detects a player above the diamond block. Initially, this works as expected and detects me only when my center is above the diamond block.
2. I then fly over to a replica of this device which is just outside of visible range from the first one. The second device is also running on a clock.
3. Then I teleport back to the first device, and you can see that its detection range has expanded to slightly larger than 1x1x1.
4. I wait a few seconds for the second device to unload, since it is now out of visible range, and the first device spontaneously returns to its initial behavior.