While reading your post a very simple explanation popped into my mind (partly thanks to misreading at first :)
The simple fact that I had missed is that with a billiard ball there actually is also a z-rotation to begin with. I had started my analysis by looking at a horizontal hit to the ball...
For an object sliding on a horizontal surface, there is a dive effect caused by friction. This means that the front edge of the object is "diving" towards the surface and the back edge has some lift. As a result the friction will be stronger in the front edge of the object, which typically...