为什么在这里打印subject.onNext（3）？

Question

在这段代码中，为什么在我设置subject.sample（500）但setTimeout（200）时首先打印subject.onNext（3）？

const Rx = require('rx');

const subject = new Rx.Subject();

const sampleObservable = subject.sample(500);

sampleObservable.subscribe(
    data => console.log(data),
    error => console.log(error),
    () => console.log('FINISHED')
);

subject.onNext(0);
subject.onNext(1);

setTimeout(() => {
    subject.onNext(2);
    subject.onNext(3);
    subject.onCompleted();
}, 200);