Minor scale issue on play mode

Hi, guys.

I'm working on a simple scene with some empties with a script that handles the rotation using the device acceleration.

This empty works as "external pivot" for the child object, so I set the scene putting this empties and setting the other objects as children, everyone with the right position and scale.

Everything is perfectly in place on the editor, but when I press play things just get slightly off.

Even if I don't touch scale factors, children objects are slightly squeezed on the x axis, but will get back to their right size if I try to manually rotate or translate them.

There is no trace of the scale variation neither on the pivot, nor on the child.

These pivots are children of objects which scale is not (1,1,1), but I don't see how this could interfere.

What could be the cause?