Some plugins create thumbnails that are not registered in the WordPress database which only exist alongside the other thumbnails.
Since ShortPixel Image Optimizer checks the database to see what thumbnails need to be optimized, if you don’t enable this checkbox, these unlisted thumbnails will not be optimized. Therefore, enable this to allow our plugin to optimize them as well.
Note: The count of images to be optimized does not include these unregistered thumbnails for performance reasons.