Unused CSS is relative, a plugin may include a large CSS file and everything is needed, but on different pages. Ideally, they would separate it… but that is what distinguish between performance centric developers and regular developers.
FVM minifies the data but it does not know which one is used or not.
If you want to remove it, you can delete the plugin that includes the file or replace it with another one that is better optimized.
If none exist, ask the developer of that plugin to reduce it.
If a plugin is not used at all 100% in a page, but it still ads the css file, you can remove it completely with Asset CleanUp plugin or similar.
If a certain percentage is in use, but the file is large, it will complain.
You can use a commercial option to uncss or hire a developer to do that.