A thumbnail in AEM Asset is defined by the width:height:center triple. Width and height define the thumbnail size in pixel. The center parameter, which is set to false by default, decides the dimensions of renditions.
If the center flag is set to true, the thumbnail image will have exactly the size given by the configuraiton otherwise it maintains the original image ratio to create renditions.
You can modify the dam update asset workflow to set the center parameter to true to get exact size renditions.