It's actually not that odd at all. A cable company only has so much bandwidth that it can display tv signals. After it reaches that point, it has to start compressing the image to supply more channels. Satellite has incorporated mpeg4 abilities which is supposed to have a comparable picture, but uses less bandwidth. That is why they can offer almost 100 stations in hd. I would debate the picture comparison heavily, though.
When using an over the air signal, (or antenna) you are receiving the picture in its truest form. There is no compression.