The bug

It appears the following selector arguments do not behave consistently:

• distance: Finds entities whose position (center of the hitbox) is within the area
• dx, dy, dz: Finds entities whose hitbox intersects with the area

This might be intended, but can also have unexpected side-effects and be unintuitive.

How to reproduce

1. Place a block at (0,0) in the floor so you know where it is
2. Show hitboxes (F3 + B)
3. Move around (0,0) and run the following command

   /say @s[x=0.5,z=0.5,distance=..0.5]
   

   It finds you as soon as the center of your hitbox is inside a circular area with center at (0.5,0.5) with radius 0.5

4. Move around (0,0) and run the following command

   /say @s[x=0.0,z=0.0,dx=0,dz=0]
   

   It finds you as soon as your hitbox intersects with the block area of (0,0)