Different results after applying ToList ()

Consider the following class:

public class Score
{
    private static readonly Guid _relationId = Guid.NewGuid();

    public Guid Id { get; set; }
    public Guid RelationId { get { return _relationId; } }

    public Score()
    {
        Id = Guid.NewGuid();
    }
}

Next function:

public IEnumerable<Score> GetScores()
{
    yield return new Score();
    yield return new Score();
    yield return new Score();
}

Why this function returns 1 :

public int Execute()
{
    var scores = GetScores();
    IList<Score> externalScores = new List<Score>();

    var filteredScores = scores.Where(score => externalScores.All(x => x.RelationId != score.RelationId));

    foreach (var score in filteredScores)
    {
        externalScores.Add(score);
    }

    return externalScores.Count();
}

But when I add .ToList()in filteredScores(either in the foreach loop or after the linq operation), the result of function 3 .

I suspect this has something to do with Closing on a loop variable , but I can't seem to figure this out.