Did not delete all unused sizes

Hi there,

Love the plugin and using the latest version of plugin and wordpress. I just noticed that if i regenerate all media. There is some images where the old sizes are not deleted. For example there used to be 15 image sizes but for some time now i fixed it and now only 5 get generated. But there are many old images that still have tons of old unused image sizes.
Why does it not delete those old sizes?

Thanks