When you press Turn off computer on the Start menu, the only buttons that are available are the Stand by, Turn Off, and Restart, even though you have hibernation enabled.
NOTE: Domain member computers, using the classic Shut Down menu, don't have these buttons.
The behavior occurs because Windows displays the Stand by button on computers that support standby. When the Stand by button is displayed on the Turn off computer dialog, the Hibernate button is NOT displayed.
To workaround this behavior, hold down the SHIFT key after the Turn off computer dialog box is displayed. This will make the Stand by button change to Hibernate.