By default your Laptop's LCD is your Primary Monitor while the external LCD is your Secondary Monitor.

You can easily verify this in the Diplay Properties | Settings tab.
Click on the button Identify and it will clearly indicate this.

Also after you have selected the second display you have to check the option Extend my Windows desktop onto this monitor

The difference in aspect ratio will mean that your external LDC will not display the entire width of the Laptop's display.