Sorry for the delayed reply, I've been away for a few days
Your not being stupid at all, it all get's very technical regarding how all the extra pixels are squeezed onto the sensor, that and plenty of other related things that I don't understand myself can cause noise to the image's it captures. One thing I have realised since I started using digital cameras is that a camera's megapixel count is not an indicator to the quality of images it produces.