java线程实现和集合类综合问题

package com.stringtest;

import java.util.HashSet;
import java.util.Set;

public class Thread1 implements Runnable {
    public void run() {
        Set set = new HashSet();
        for (int i = 0; i < 10; i++) {
            set.add((int)(Math.random()*100));
        }
        System.out.println(set);
        System.out.println(set.size());
    }
}
线程实现代码
*********************

package com.stringtest;

import java.util.HashSet;
import java.util.Set;

public class Poeal {
    public static void main(String[] var12)  throws InterruptedException{
         Thread1 thread1 = new Thread1();
         for (int i=0;i<12;i++) {
             System.out.println("线程第"+i+"次启动");
             thread1.run();
             Thread.sleep(1000);
         }
    }
}
线程运行代码
***********************
运行结果

线程第0次启动
[16, 32, 80, 96, 84, 85, 71, 89, 47, 95]
10
线程第1次启动
[17, 33, 66, 19, 84, 53, 75, 12, 76, 62]
10
线程第2次启动
[66, 50, 82, 67, 69, 88, 75, 60, 77, 15]
10
线程第3次启动
[82, 50, 69, 85, 87, 61, 13, 93, 79, 31]
10
线程第4次启动
[80, 65, 82, 83, 67, 5, 7, 25, 94, 79]
10
线程第5次启动
[32, 4, 6, 38, 87, 56, 25, 59, 28, 46]
10
线程第6次启动
[16, 64, 65, 4, 9, 25, 90, 91, 13]
9
线程第7次启动
[96, 51, 53, 7, 8, 42, 10, 59, 43, 60]
10
线程第8次启动
[64, 17, 81, 19, 35, 23, 12, 77, 29, 47]
10
线程第9次启动
[80, 32, 33, 82, 98, 3, 69, 92, 93, 45]
10
线程第10次启动
[82, 4, 21, 6, 39, 88, 9, 43, 44, 31]
10
线程第11次启动
[2, 3, 83, 53, 38, 6, 24, 9, 11, 92]
10

Process finished with exit code 0

问题：

为何有一次结果，输出为9呢