http - Why this simple web server is called even number times?

Question

I'm trying to learn Go web programming, and here is a simple web server: it prints out the times being called.

package main

import (
  "fmt"
  "net/http"
)

var calls int

// HelloWorld print the times being called.
func HelloWorld(w http.ResponseWriter, r *http.Request){
    calls++
    fmt.Fprintf(w, "You've called me %d times", calls)
}

func main() {
  fmt.Printf("Started server at http://localhost%v.
", 5000)
  http.HandleFunc("/", HelloWorld)
  http.ListenAndServe(":5000", nil)
}

When I refresh the page, I got:

You've called me 1 times
You've called me 3 times
You've called me 5 times
....

Question: Why it is 1, 3, 5 times, rather than 1,2,3...? What's the order of the function HelloWorld being called?

Answer 1

Waitting for answers