Mac folder which is Not Shared is shared and fully accessible from a Windows machine
According to this answer on the Apple Support forum, if you enable (tick) a user for SMB sharing, it shares the root of the system drive (provided they are an Admin): 


  Users you have set up with accounts in Accounts preferences can access
  your computer by connecting to it over the network. A user with an
  administrator account can access your entire computer.


You can see the drive because you are an administrator, and you entered your password on the windows machine. If you hadn't entered your password (or had used a non-administrator account) you wouldn't have been able to see the drive.