Star Choices dishes are oval because (like Directv) they have more than one bird in the sky at different oribital slots.
If you see a picture of the satellite, you will notice different panels. Each panel is a transponder. Directv/BEV/DN uses 32 transponders (they have more in case one goes puff).
The more transponders you are using mean the more channels you can transmit. The FCC (in the states) dictates the amount of bandwidth that can be used at each slot.
Different channels are on different transponders. Each one has a different signal strenght. During rain some channels will be fine and others will have pixelation or be out of commision (until the rain stops)