用 go 写 worker pool 的疑惑

研究用 go 控制并发数的时候发现一个example，就改造了下发现一个问题，源码

package main

import "time"

func worker(id int, jobs <-chan int, results chan <- int) {
	for j := range jobs {
		time.Sleep(time.Second)
		results <- j
	}
}

func main() {
	jobs := make(chan int, 100)
	results := make(chan int, 100)
	for w := 1; w <= 3; w++ {
		go worker(w, jobs, results)
	}

	//发送 jobs
	for j := 1; j <= 10000; j++ {
		go func() {
			jobs <- j
		}()
	}
	//go func() {
	//	for j := 1; j <= 10000; j++ {
	//		jobs <- j
	//	}
	//}()

	for {
		select {
		case r := <-results:
			println("result:", r)
		}
	}

}

但输出缺有缺失，并且还有重复

result: 14
result: 15
result: 8
result: 15
result: 15
result: 15
result: 15
result: 15
result: 15
...

将源码中发送 jobs 的部分改为

	go func() {
		for j := 1; j <= 10000; j++ {
			jobs <- j
		}
	}()

结果又是正常的

result: 3
result: 2
result: 1
result: 5
result: 4
result: 6
result: 9
result: 8
result: 7
result: 10
result: 12
result: 11
result: 14
result: 15
result: 13

有谁知道原因吗？