Hi, 100% scale just means that no uniform scaling is applied to the image.
It is easiest to understand this by (for now) ignoring the Vertical, Horizontal, and Rotate sliders. When you leave Scale at 100%, you see the entire image area. When you reduce it to 99%, you start seeing the gray padded border. Similarly, when you expand it to 101%, you start dropping image data as it's cropped off.
Now, when you start using the other sliders (Vertical, etc.), this concept remains unchanged. The only thing that's changed is that the gray padded area is no longer uniform around the image edges, and the original image area is no longer axis-aligned, nor even rectangular. This is because the perspective projection can be arbitrary. Practically, the Scale slider is used to help you decide how much of this "warped" image area you wish to retain.