Selectors dx, dy, and dz are bugged

When you try to select a area with testfor, it selects seemingly infinitely in the positive x, positive y and positive z directions.

Steps to recreate:
1. Get a command block.
2. Enter a testfor command in the command block using dx, dy, and dz.
3. Build a clock next to the command block to pulse it.
4. Go off in the east, south, and up directions.