SharePoint site Sharing permissions

Hi.

I created a site to be used like a library. 

I set up myself as Owner, and everybody else is a Visitor.

Within Site Sharing Setting I selected the second option "Site owners and members and people with Edit permissions can share files and folders, but only owners can share the site". I asked one of my colleagues to try sharing a file, and though they are set up as visitors (Read) they are still able to share them.

How is that possible please?

thanks