ShortPixel shows that I have 500 images but in Media Library I only have 100 images, how comes?
ShortPixel counts each image and thumbnail separately.
That means that if you have 100 images but each image has 4 associated thumbnails you'll have a total of 500 images that ShortPixel can optimize. Please note that sometimes, depending on the themes/plugins installed you can have even 50 (sic!) or more thumbnails associated with each image. In that case ShortPixel will count 5000 thumbnails and 100 images = 5100 images&thumbnails that can be optimized!
Please note that 500 and 100 figures above are just an example but the rule works in general for any WordPress installation.