Why, when generating random numbers, the number of which corresponds to the maximum size of the number, unique numbers are always 0.63 of the size?

package com.company; import java.util.*;

public class Main {

public static void main(String[] args) {
    ArrayList<Integer> integers = new ArrayList<>();
    for (int i = 0; i < 20000000; i++) {
        integers.add((int) (Math.random()*20000000));
    }
    HashSet<Integer> unique = new HashSet<>(integers);

    System.out.println("Number of items in a list of numbers: " + integers.size());
    System.out.println("Number of unique elements: " + unique.size());
}
}