Image written to disk and retrieved are not the same

When I write an image/frame to disk using write using cv::VideoWrite (creating a small movie) and read same image/frame from disk (cv::VideoRead) and compare them by subtraction: they are not exactly the same. It seems to happen in the 'red' channel with differences of around ~10-30 (split channels and subtract). Any Idea why this happens and what to do about it?? see for example testcode: [link text](http://stackoverflow.com/questions/33438156/images-are-not-the-same-after-videowrite-and-videoread-opencv)