optical illusion:
It really
is curved top to bottom.
Think of a four foot dish, with a slice about a foot high taken out of the middle (top and bottom are gone).
The slight amount of curve is hard to see, but I assure you it
is a slice of a parabola!
All the energy of the height of the dish is focused onto a feedhorn about four inches high.
Below is a picture of something similar (not the real deal) to show what I'm talking about.
Just don't have a good close up of the feed on the dish I posted above.
edit:
The four foot width, gathers lots of signal from the desired satellite, and suppresses signals from adjacent birds.
So, for all intents and purposes, it works
as well in this application, as a big four foot round dish would.