This would indicate why it tends to affect it by only a few % points, and actually seems to me to make it a directional antenna.
Even one layer of metal ( not a cage) between an antenna and transmitter will dampen signal (or distort it) Even if the metal re-radiates the signal it will be lessened.
The only other possibility would be that the engineers could have actually used conductive plastic to use the cover as the antenna I think that this would have been possible; however I see to evidence that this is the case.

-Ezra Hilyer