I also found that when the weather is bad the sync of sound and picture goes. If you zap up or down one channel and then back, this will normally fix the problem. Also if you are watching live TV the sync can be way out. As to why this happens, beats me.
The sync problem can be generated at one or both ends of the link
PaulR is correct that the compression used can affect the way sound and vision syncronise in the datastream, however the receiver in the house has much work to do by resolving the two outputs to the satisfaction of the viewer, by virtue of the information supplied with the data, which may have errors as a result of low signal (poor weather), increased noise (misaligned or too small a dish/leaky LNB/cable). Then there is the receiver firmware itself - many people will know the problems that continue to plague the cheaper end of the receiver market and likewise Manhattan 2500 and 5500 receivers, the lips move but the sound follows a second later, listening to classical music on these receivers can be a painful experience