It all depends on both list and how they are compared.

For example
(list1.Contains(wordFromList2)) == (list2.Contains(wordFromList1))
but
(!list1.Contains(wordFromList2)) != (!list2.Contains(wordFromList1))


Here is a quick example illustrating what I mean.
Code:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            // Create 2 list
            string[] exampList1 = { "fruit", "friend", "dog", "cat", "people" };
            string[] exampList2 = { "fruit", "cat", "rat", "human", "pie"};

            Console.WriteLine("Example 1:");
            Console.WriteLine("Strings from list 1 found in list 2\n");

            // Loop goes through all strings found in list
            foreach (string s in exampList1)
            {
                // Checks if string is found in the other list
                if (exampList2.Contains(s))
                {
                    // Writes out the string
                    Console.WriteLine(s);
                }
            }

            Console.WriteLine("\n\nExample 2:");
            Console.WriteLine("Strings from list 1 not found in list 2\n");

            // Loop goes through all strings found in list
            foreach (string s in exampList1)
            {
                // Checks if string is not found in other list
                if (!exampList2.Contains(s))
                {
                    // Writes out the string
                    Console.WriteLine(s);
                }
            }

            Console.WriteLine("\n\nExample 3:");
            Console.WriteLine("Strings from list 2 found in list 1\n");

            // Loop goes through all strings found in list
            foreach (string s in exampList2)
            {
                // Checks if string is not found in other list
                if (exampList1.Contains(s))
                {
                    // Writes out the string
                    Console.WriteLine(s);
                }
            }

            Console.WriteLine("\n\nExample 4:");
            Console.WriteLine("Strings from list 2 not found in list 1\n");

            // Loop goes through all strings found in list
            foreach (string s in exampList2)
            {
                // Checks if string is not found in other list
                if (!exampList1.Contains(s))
                {
                    // Writes out the string
                    Console.WriteLine(s);
                }
            }

            // Wait for user input before closing application
            Console.Write("\n\nPress any key to continue...");
            Console.ReadKey();
        }
    }
}