hashmap related questions

I am checking below link on hashmap

https://www.youtube.com/watch?v=iinE6ZBNBjw

package test;

import java.util.HashMap;

public class TestHashmap {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		HashMap<String, Integer> map = new HashMap<>();
		map.put("a1", 1);
		map.put("a2", 2);
		map.put("a3", 3);
		map.put("a4", 4);
		System.out.println("map--" + map);
		//String str1
	}

}

Select all Open in new window

when i put 'a1' why it put in 8th bucket out of all 16 buckets rather than 1st. Why it put second element in 9th bucket.