Because you are using the extend mode. In extend mode, the Coast 12 is an extension of your computer. To correctly use Coast 12 in extend mode, follow the tips below:
1. Right click your desktop to enter Display settings.
2. There are 1 and 2 in the top, click 2 which represents Coast 13. You could click Identify to know what 1 and 2 separately represent.
3. Make it the "main display" in the bottom.
4. If you want to make Coast 12 show same as your computer, change “Extend these displays” to “Duplicate these displays” in Multiple displays.
1. Go to System Preference-Displays-Arrangement.
2.Make Coast 12 the main display: move the “White head” from your Mac to Coast 12.
3. If you want to make Coast 12 show same as your Mac, click Mirror Displays.