If you are still using the "older" model Dual View/PVR Decoders, then you will have this problem.
I also experience this, and phoning DSTV, they informed me that only certain channels are 16:9, so it is best to set up the decoder for normal 4:3 ratio.
They have no plans to fix this, e.g. SW Upgrade and the only remedy for this is to buy a HD Decoder.
Talk about sales pitch!!