C#, For Loops, and speed test… Exact same loop faster second time around

public Int64 ReturnDifferenceA()
{
  User[] arrayList;
  Int64 firstTicks;
  IList<User> userList;
  Int64 secondTicks;
  System.Diagnostics.Stopwatch watch;

  userList = Enumerable
              .Range(0, 1000)
              .Select(currentItem => new User()).ToList();

  arrayList = userList.ToArray();

  watch = new Stopwatch();
  watch.Start();

  for (Int32 loopCounter = 0; loopCounter < arrayList.Count(); loopCounter++)
  {
     DoThings(arrayList[loopCounter]);
  }

  watch.Stop();
  firstTicks = watch.ElapsedTicks;

  watch.Reset();
  watch.Start();
  for (Int32 loopCounter = 0; loopCounter < arrayList.Count(); loopCounter++)
  {
     DoThings(arrayList[loopCounter]);
  }
  watch.Stop();
  secondTicks = watch.ElapsedTicks;

  return firstTicks - secondTicks;
}

differenceList = Enumerable
                 .Range(0, 50)
                 .Select(currentItem => ReturnDifferenceA()).ToList();
average = differenceList.Average();

differenceListA = Enumerable
                  .Range(0, 50)
                  .Select(currentItem => ReturnDifferenceA()).ToList();
averageA = differenceListA.Average();

differenceListB = Enumerable
                  .Range(0, 50)
                  .Select(currentItem => ReturnDifferenceA()).ToList();
averageB = differenceListB.Average();