The selectors dx, dy, dz don't give the box one would expect; instead, the selection box seems to distended by one block in the positive x and z directions (and possibly in the y direction, though I did not test this).

The setup is given in the first screenshot—I surrounded a zombie by armor stands at 0.1 block intervals in the four coordinate directions, then ran /execute as @e[type=zombie] at @s run kill @e[dx=0,dy=0,dz=0,type=armor_stand].

Expected: only the center stands would be killed, symmetrically, as only their hitboxes would contain the coordinates of the zombie.

Actual: the curious pattern shown in the second and third screenshots was produced, in which the volume box seems correct in the negative X and Z directions but too large in the positive ones.

Running /execute as @e[type=zombie] at @s positioned ~-0.5 ~ ~-0.5 run kill @e[dx=1,dy=1,dz=1,type=armor_stand] produced a similarly distorted pattern, shown in screenshot four.