Next Method (Int32, Int32)

This program is given in the following website.
http://msdn.microsoft.com/en-us/library/2dx6wyd4.aspx

When calculating random integers, in Next() method always 1 is added to the upper limit. Please tell me the reason. Problem is highlighted.

using System;

public class Example
{
public static void Main()
{
Random rnd = new Random();

Console.WriteLine("\n20 random integers from -100 to 100:");
for (int ctr = 1; ctr <= 20; ctr++)
{
Console.Write("{0,6}", rnd.Next(-100, 101));
if (ctr % 5 == 0) Console.WriteLine();
}

Console.WriteLine("\n20 random integers from 1000 to 10000:");
for (int ctr = 1; ctr <= 20; ctr++)
{
Console.Write("{0,8}", rnd.Next(1000, 10001));
if (ctr % 5 == 0) Console.WriteLine();
}

Console.WriteLine("\n20 random integers from 1 to 10:");
for (int ctr = 1; ctr <= 20; ctr++)
{
Console.Write("{0,6}", rnd.Next(1, 11));
if (ctr % 5 == 0) Console.WriteLine();
}
Console.Read();
}
}