Why is in C# a nested foreach loop more performant than a SelectMany combined with a single foreach loop or a GroupBy?

Question

Consider having these Models:

public class BenchMarks{
    public List<Parent> Parents { get; set; }
}

public class Parent {
    public List<Child> Children { get; set; }
}

public class Child {
    public string Name { get; set; }
    public int Value { get; set; }
}

and the class BenchMarks also has these methods:

public void GetWithTwoForeach(){
    var dict = new Dictionary<string,int>();
    foreach(var parent in Parents){
        foreach(var child in parent.Children){
            if (dict.ContainsKey(child.Name))
                dict[child.Name] += child.Value;
            else
                dict.Add(child.Name, child.Value);
        }
    }
}

public void GetWithOneForeach(){
    var dict = new Dictionary<string,int>();
    foreach(var child in Parents.SelectMany(p=>p.Children)){
        if (dict.ContainsKey(child.Name))
            dict[child.Name] += child.Value;
        else
            dict.Add(child.Name, child.Value);
    }
}

public void GetWithGroupSum(){
    var childGroups = Parents.SelectMany(p=>p.Children).GroupBy(c => c.Name);   
    var dict = childGroups.ToDictionary(cg => cg.Key, cg => cg.Sum(child => child.Value));
}

Than why is it that the nested foreach loop outperforms the other two methods by more than 100%?

Benchmark fiddle

Edit My benchmark was indeed affected by the ordered nature of my test data.

When I changed it to this (I was not able to get this to work in the .NET fiddle)

public void Setup()
{
    Parents = new List<Parent>();
    
    var names = new List<string>{"Tigo","Edo","Kumal","Manu","Guido","Anne","Steff","Mimi","Suzan","Jeff"};
                    
    var random = new Random();
    
    for (var i = 0; i < 200; i++)
    {
        int index = random.Next(names.Count);
        Parents.Add(new Parent
        {
            Children = new List<Child>
            {
                new Child
                {
                    Name = names[index],
                    Value = i
                }
            }
        });
    }
}

The benchmark came out a bit less dramatic, but still the same conclusion that LINQ adds overhead as mentioned in comments