Some confusion about safepoint and -XX:GuaranteedSafepointInterval

My env:

1. JDK : temurin-1.8.0_332
2. System: macOS big sur
3. VM: Hot Spot
4. GuaranteedSafepointInterval = 1000ms (default)

Q1: The vm param GuaranteedSafepointInterval = 1000ms is really accurately?

    public class Test {

    public static AtomicInteger count = new AtomicInteger(0);

    public static void main(String[] args) throws InterruptedException {
        Runnable task = () -> {
            for (int i = 0;i < 1000000000;i++){
                count.getAndAdd(1);
            }
            System.out.println(Thread.currentThread().getName() + " done" );
        };

        Thread t1 = new Thread(task);
        Thread t2 = new Thread(task);
        t1.start();
        t2.start();
        Thread.sleep(1000);
        System.out.println("count result = " + count);

    }
}

As expect,because of the safepoint, the result is

count result = 2000000000
Thread-1 done
Thread-0 done

But ,if change the main thread sleep time to 800ms...(more you can use <800ms) result

So, the default value GuaranteedSafepointInterval = 1000ms is really accurately?

Q2: Sometimes,if i still set sleep time = 1000ms,but change the loop content count.getAndAdd(1) to count.getAndAdd(i) ,the "count result" will print after 1s...... as i know,the hot spot vm(jdk=1.8) does some optimizations for 'Counted Loop' when sets savepoint ,(according to the Q1 scence),why it doesnt effective？

Any Can Help Me? More Thanks...