In the context of concurrent execution, why might the counts array not reflect the expected values after both threads execute?

public class AtomicMistake {
    private int[] counts = new int[2];
    
    public void incrementBoth() {
        counts[0]++;
        counts[1]++;
    }
    
    public static void main(String[] args) {
        AtomicMistake am = new AtomicMistake();
        Runnable task = () -> {
            for (int i = 0; i < 1000; i++) {
                am.incrementBoth();
            }
        };
        Thread t1 = new Thread(task);
        Thread t2 = new Thread(task);
        t1.start();
        t2.start();
    }
}