Rect IsEmpty doesn't return true where width or height is zero

In C# IsEmpty (on System.Windows.Rect) doesn't return true where Width or Height is zero. In MSDN it is noted but not (clearly) explained:

Do not use this property to test for zero area; a rectangle with zero area is not necessarily the Empty rectangle. For more information, see the Empty property.

Why isn't IsEmpty implemented as:

public bool IsEmpty
{
    return Width == 0 || Height == 0;
}

What is the use case for the current implementation?