This addon does not crop the images to exact sizes... instead it resizes the image proportionally. I would guess that your original image has 4:3 aspect ratio, so setting thumbnail width and height to 100px will result in a resize of the image down to the max width of 100px, then the height gets adjusted proportionally to maintain the original aspect ratio.

If you want to be able to crop the images to a specific size, try the Deluxe Image Gallery addon:http://www.concrete5.org/marketplace/addons/deluxe-image-gallery...