Hi @triptikon
Yes, it is correct.
Let me extend a bit the explanation.
When we upload images to our website WordPress will generate different thumbnails, and themes and plugins may register their own sizes too.
A full explanation can be found at: https://wpmudev.com/blog/wordpress-image-sizes/
Smush on the free version will compress those thumbnails and as Adam explained the stats means the total saving of all those versions, so the plugin is working as expected.
We can’t also force further optimization in an image that is already optimized, Smush will focus on optimization without big quality loss so the amount of saving depends on images too.
Best Regards
Patrick Freitas