RXJS的最新行为异常

Question

我正在遵循教程（https://www.learnrxjs.io/operators/combination/withlatestfrom.html）中的示例：

// RxJS v6+
import { withLatestFrom, map } from 'rxjs/operators';
import { interval } from 'rxjs';

//emit every 5s
const source = interval(5000);
//emit every 1s
const secondSource = interval(1000);
const example = source.pipe(
  withLatestFrom(secondSource),
  map(([first, second]) => {
    return `First Source (5s): ${first} Second Source (1s): ${second}`;
  })
);
/*
  "First Source (5s): 0 Second Source (1s): 4"
  "First Source (5s): 1 Second Source (1s): 9"
  "First Source (5s): 2 Second Source (1s): 14"
  ...
*/
const subscribe = example.subscribe(val => console.log(val));

当我使用RXJS 6.2.2，Node 8.9.4运行此代码时，我期望的是：

First Source (5s): 0 Second Source (1s): 4
First Source (5s): 1 Second Source (1s): 9

但是我得到了

First Source (5s): 0 Second Source (1s): 3
First Source (5s): 1 Second Source (1s): 8

这是为什么？