I had an image on my website and I placed a caption on it. When I went to change the caption background color with the following code:
The caption block would be several pixels wider on the right side than the image or than the amount of padding on the top and left. I have since removed that particular image as it was a problem with the display. The problem is remedied if the image is wider than the width of the screen so the caption is only a few pixels wider if the image is less wide.
EDIT: I also just noticed that the image is not centered when caption is user and actually is slightly right of center.