By binding a dropdown to a variable, it would make sense for that variable to hold the selected option of the dropdown at all times. However, this is not the case. If you select an item in a dropdown, then refresh the list of items (via a variable in ng-options), if the selected item is not included in the new list it reverts back to the default option (visually) while ngModel bound variable holds the old selected value.

Here is a fiddle: http://jsfiddle.net/LUm7y/