Looks like you are using lazy loading for images and the HiDPI add-on. So some of the thumbnails for the HiDPI devices are twice the size, 1280x800. Please check that the lazy thumbnail limits in config.php are higher than the HiDPI thumbnail size. The default values are:
$config['lazy_thumbnails'] = array(
'max_width' => 1280,
'max_height' => 720
);
So checking from the default config, image height 800 is clearly larger than the limit of 720, so the thumbnail won’t be generated in this case.