Super-Smush Images issues with the WP Smush

Hi there,

We have a problem with WP Smush Pro. It did not work or behave all the same way. We wonder why. Our client has some websites, when I tried to use this plugin to compress their images, one site is working fine. Notice, for each single site, my client has almost an identical media library, I mean the images are the same when the previously the sites were built up. Quite surprisingly, one of her website's images were compressed greatly, which the toal image sizes reduced by 26.05 MB. However, after we re-visit WP Smush Pro inside the media library, it shows us a message for Super-Smush Images, stating that they found 2 attachments that were previously smushed losslessly. If desired we can Super-Smush them now for more savings with almost no noticeable quality loss.
Ok, we click the button, and have another go at Super-Smush. after a few seconds, job is done. Then we refresh the browser, again another message stating that they found 2 attachments that were previously smushed losslessly. If desired we can Super-Smush them now for more savings with almost no noticeable quality loss.
We gave another go at it, then refresh the browser after, the message shown again and again, it seems the plugin is not trying to do a buck smush, we will have to do it many times? Is this normal? We are confused.
Another issue raised is that since my client has all the media library stored identical images, then the result of compression shall be the same too. However, we found the plugin reduced the images size respectively by 26.05 MB, 2.41 MB, 334.3 KB(with an error message: 89 image(s) were skipped due to an error), 3.9 MB and 24.9 MB.
But noticeably, all of the media library has almost the same number of identical images. How will it reduce all the image size so differently?
At moment, we are very confused, please help. thanks