I find it works loose as well after some use. I think it is the design of the CD slot mouth (at least in the 2011). I have to tighten the mount a fair bit to secure it. This much extension on the screw seems to deform the lower edge of the unit.
Here are some pics:
CD slot (2011).. note how it's wide vertically before narrowing inside:
The mount in place:
The mount tightened to where it was snug in place. Note the deformed shape of the lower edge:
Here it is with the tightening screw fully retracted (no tension).. notice how the lower edge plastic of the unit keeps some of its deformed shape: