Webflow is using cascading CSS styles. It is about styling objects for different viewports.
If you will look inside the exported CSS files, you will see that there is different media query (@media screen .... ) for different screen sizes.
It means that originally style properties and values will be saved for desktop screen size and they will be used for smaller screens /viewports if there will be not applied changes for the particular viewport. So all style changes that you are making for different views in designer mode going to be saved under different media queries.
Hope I was able to explain.