Why Box Colliders 2D does not touch ?

Hello everybody,
I was playing around with Unity 2D physics engine and I was wondering why the colliders slightly float upon each other.

Here is what I’m talking about (Scene view while the game is running):

As you can see the Blue square floats above the Yellow rectangle altough the physics engine acts as they are colliding.

The Blue square has a Rigidbody 2D and a Box Collider 2D while the Yellow rectangle has just a Box Collider 2D and everything has the Transform scale set to X: 1, Y: 1, Z: 1, the Blue square sprite has a size of 32x32 pixels and the Yellow rectangle sprite has a size of 128x16 pixels.

I’ve made a project from scratch just to test this, so everything I am using has its default values, even the sprites have been imported with Unity default values (Pixels per Unit: 100).
I’m using Unity 4.6.2f1

Is there something I am doing wrong ? Is it fixable or is this working as designed ?

Thank you for your help !

This error is do to the way the physics engine works, if you adjust the bounds of your box collider to be slightly smaller than your sprite. It will not be noticeable.