Prior to Mac OS X Lion, Mac OS X Client came with SAMBA Server and Client and what you defined to share was all that was shared. Since Mac OS X Lion, and continuing with OS X Mountain Lion and other releases of OS X, SAMBA has been replaced with SAMBX due to SAMBA moving to GPLv3. As a result SAMBA is basically broken in the OS since 10.7. The Mac OS X 10.7 Server and OS X Server going forwards does have some finer sharing controls not present in the Client version of the OS.
As a result of these changes the Macintosh HD and User's Home folders are automatically shared for them as an Administrative User and only the Home folder as a Standard User, in conjunction to what one chooses to share in System Preferences > Sharing > File Sharing.
To see the additional shares including what's defined in System Preferences > Sharing > File Sharing and what's not defined there. In a Terminal, the output below shows what's shared for me as an Administrative User and only sharing a folder named Temp in File Sharing. With File Sharing enabled in System Preferences > Sharing, just copy and paste the following command into Terminal and press Enter:
smbutil view //$USER@$HOSTNAME
$ smbutil view //$USER@$HOSTNAME
Password for $HOSTNAME:
Share Type Comments
Macintosh HD Disk
4 shares listed
If you do not want those shares exposed then do not turn on File Sharing via normal System Preferences and instead install SAMBA and manually configure file sharing yourself.