Currently the @a[dx,dy,dz] and @a[r] selectors match the player when their hitbox first enters the bounds. This is counter-intuitive, as the player can be in a certain block (as denoted by Block on F3) but can trigger a selector for a different block.

The player's world collision hitbox is a cylinder with a radius of 0.3 from what I can tell, which means a player can be up to 0.3 blocks away from a block that is within the selector's range and still cause the selector to select them. In certain situations you can also walk into a solid block that is within the selector field and still be matched, as it seems you sometimes clip into them slightly.

I've attached a video demonstrating this with the cubic selector, although it seems to effect the radius selector too. You can see that the cubic selector matches me if I stand on the comparator next to the command block, as it seems I clip slightly into the command block.

Selecting the player when their hitbox enters the area may be the intended functionality, however clipping into blocks and being matched should not happen.