Best question so far...
The antenna is a beam forming antenna and the beam actually moves within the antenna electronically, not physically. The beam is at an angle to the flat surface; thus, the antenna physical mount is angled for the best reception of the transmission signal. It's not North or South, etc., it's the beam forming.
This video is for geeks but gives a good visual on how the antenna is transmitting and receiving and shows the angled output of the beam. It's long and somewhat high-tech talk, but very informative. (Tongue in cheek advertisements as well.)
Grab some popcorn to stay awake.