Hi visionitse,
if you delete the thumbnails and generate them again the plugin will count them as new compressions.
If you generate thumbnails for the first time (even if its from an old image), the plugin will also count them as new images. The plugin takes into account all the image sizes selected in your WordPress media settings (Media >> Settings) and compress each one of them.
For example if you upload 1500 images with 5 image sizes selected in your media settings, the plugin will compress a total of 7500 images (5 x 1500). If you decide for any reason to delete these images, re-upload and compress them, the plugin will indeed count them as new compressions.
The plugin will count a compression for each new size selected/added in your media settings. Although once a thumbnail of a certain size has been generated and compressed for an image (and it is not deleted from your media library), then the plugin will only count it as 1 compression.
Hope that answers your question! If not please send us an email at [email protected] and we can send you a more detailed explanation.