SImple bouncing ball with pysics : wierd and not logical bounce

Hello,

as you can see in this video: http://youtu.be/pTdWWI5y_IU , I've problem with bounce feature on the ball. The project is made with - 4 plans as walls (up, bottom, left, right) : no physics material - 1 ball with a physic material (bounce at 1) and a rigidbody - 2 cube as front and back walls - a cube for the paddle (but the bounce on paddle is ok)

I've read a lot on bouncing problem but as the project is the simple it could be (1 ball with rigid body, no gravity, and 4 walls) I can not figure out why I have this wierd bounce.

Thanks for you help

François