小白求教，使用goroutine為啥每次執行結果不一樣？

package main

import (
    "fmt"
    "runtime"
)

func say(s string) {

    fmt.Println("aa")
    fmt.Println(s)
    runtime.Gosched()
}
func main() {
    go say("world")
    say("hello")
}

λ go run index0.go
aa
hello

E:\golang\goroutine (master)
λ go run index0.go
aa
hello
aa

E:\golang\goroutine (master)
λ go run index0.go
aa
world
aa
hello