I could be wrong, but my understanding has always been that 100% is any size crop of an original image where the pixel ratio is 1:1 - in other words every pixel (within the cropped area) taken is displayed on the screen. Typically images displayed on a screen (like many of the ones on this forum) have been resized so the who image can be seen without panning/scrolling. When an image has been resized you are only seeing a fraction of the actually pixels that where captured by the image sensor. Hope that helps clarify.