This radio stuff can be pretty mysterious, (they're discovering new things all the time) but mostly because of the tricky way radio waves can cancel each other out, from reflections off metal or absorption by solid stuff. This means that reception can be fine in one spot and lousy in the spot right next to that spot; It depends on the frequency, the higher the frequency the closer the spots can be.
So, having two antennas rules out most all of the bad spots. This is usually only a problem when you get towards the limit of your range, but it can happen anytime.
The phony diversity, or not-so-true-diversity receivers also have two antennas. But they only have ONE receiver. They pick the antenna with the loudest signal and receive with that one. Trouble is, the loudest signal DOESN'T HAVE TO BE YOURS.