The image is displayed in a zoom percentage other than 100%
You will enter the custom mode as soon as you change the zoom factor. By pressing keyboard shortcut [+] or [-] you will either zoom-in or zoom-out all boxes. By holding the [Ctrl] key pressed and then [+] or [-] you can zoom the selected box only in or out.
The zoom steps will allow you to zoom from 1% to 5000%.
Another way of zooming the box in or out is by using the mouse-scroll button. Place the mouse over any box and use the scroll button to zoom up or down. If you hold the [Ctrl] key and then scroll the mouse up/down you will zoom just the selected box in and out.