Try it with an aluminium or magnesium Dell and you'll still be at 70%.
Dell puts plastic windows by the radio antennae so that you'll get good reception regardless.
I know for a fact that Powerbook and Macbook Pro's up until the Unibodies had plastic "windows" as well. In powerbooks I believe it was on the sides of the display, and on the MBP's it was in the hinge mechanism for the display. Not sure about the uMBP's, but I don't see why apple wouldn't continue this trend. Their engineers are sure to know as good as everyone that metal of most kinds is a poor enclosure to encase a RF antenna in. If they did indeed give up on the "window" then shame on them for deciding such amount of signal loss is "OK"...