Random generates number 1 more than 90% of times in parallel

Consider the following program:

public class Program
{
     private static Random _rnd = new Random();
     private static readonly int ITERATIONS = 5000000;
     private static readonly int RANDOM_MAX = 101;

     public static void Main(string[] args)
     {
          ConcurrentDictionary<int,int> dic = new ConcurrentDictionary<int,int>();

          Parallel.For(1, ITERATIONS, _ => dic.AddOrUpdate(_rnd.Next(1, RANDOM_MAX), 1, (k, v) => v + 1));

          foreach(var kv in dic)
             Console.WriteLine("{0} -> {1:0.00}%", kv.Key, ((double)kv.Value / ITERATIONS) * 100);
     }
}

This will print the following output:

(Note that the output will vary on each execution)

> 1 -> 97,38%
> 2 -> 0,03%
> 3 -> 0,03%
> 4 -> 0,03%
...
> 99 -> 0,03%
> 100 -> 0,03%

Why is the number 1 generated with such frequency?