Isso é explicado na documentação oficial do ImageMagick, em “Cortar a miniatura para caber” :
An alternative, is rather than pad out the image to fit the specific thumbnail size we want, is to instead cut off the parts of the image that does not fit the final size.
Of course this means you actually lose some parts of the original image, particularly the edges of the image, but the result is a enlarged thumbnail of the center part of the image. This is usually (but not always) the main subject of the image, so it is a practical method of thumbnail creation.
As of IM v6.3.8-3 the special resize option flag '^' was added to make this easier. We just resize using this flag then crop off the parts of the image that overflows the desired size.
E no contexto de um exemplo de comando:
convert -define jpeg:size=200x200 hatching_orig.jpg -thumbnail 100x100^ \
-gravity center -extent 100x100 cut_to_fit.gif