ADB does not reboot my Samsung Galaxy SIII I9300 to bootloader (download) mode

My device is Samsung Galaxy S3 I9300 and I did a complete factory reset. The device is **NOT** rooted yet. It works properly. I can do all of the following with my device: * Power off using device keys. * Start in **recovery mode** and **download mode** using device keys. * device is detected properly in **normal mode** and **recovery mode** by adb when is connected to PC through a USB cable * I am able to reboot the device using adb reboot command * adb reboot recovery works well. The problem is that if I run adb reboot bootloader or adb reboot-bootloader commands, the device is booted in normal mode, NOT in **download mode**. The only way to reboot the device in **download mode** is to use the **Power + Volume Down + Home** keys