测量课堂上的功能之间的时间？

I am trying to understand OOP and made my own class to benchmark a MySQL DB

class Benchmark
{

    protected $sql, $db;
    public $result, $time;

    public function __construct($host, $user, $pass, $db, $result = array(), $time = null)
    {

        /* Vars */
        $this->db = $db;
        $this->result = $result;
        $this->time = $time;

        /*Start Timer */
        $this->time = microtime(true);

        /* Connect to DB */
        $this->sql = new \mysqli($host, $user, $pass);

        /* Measure Time and put result to array */
        $this->result['benchmark']['connect'] = $this->elapsedTime($this->time);


    }

    public function testMySQL()
    {

        /* Connect to DB */
        $this->sql->select_db($this->db);
        $this->result['benchmark']['selectDb'] = $this->elapsedTime($this->time);

        /* Fetch Version */
        $version = $this->sql->server_version;
        $this->result['benchmark']['version'] = $this->elapsedTime($this->time);
        $this->result['info']['version'] = $version;

        /* Benchmark */
        $this->sql->query('SELECT BENCHMARK(1000000,ENCODE("hello",RAND()));');
        $this->result['benchmark']['result'] = $this->elapsedTime($this->time);

        /* Close Connection */
        $this->sql->close();

        /* Total Time */
        $this->result['info']['total'] = $this->elapsedTime($this->time);

        $this->dump($this->result);

    }
}

However, the result is this:

Array
(
[benchmark] => Array
    (
        [connect] => 0.001
        [selectDb] => 0.001
        [version] => 0.001
        [result] => 10.181
    )

[info] => Array
    (
        [version] => 50713
        [total] => 10.181
    )

)

Why doesnt the time add up? Shouldn't [total] be 10.184 in this case?

The original idea is from here :https://github.com/odan/benchmark-php and that works. If I use the same data on that script, it adds up, where am I wrong? I guess it has something to do with my lack of OOP though...

EDIT

 public function elapsedTime($time)
 {
     return number_format(microtime(true) - $time, 3);
 }

If you are measuring the elapsed time since the timer is started, then each output is simply a point in that time period. The time it takes from initializing the timer until the result is obtained is not in addition to the time it takes to connect, select the db, and get the version, those are merely checkpoints along the way (which appear to be occurring virtually simultaneously, at least at the resolution of this timer). So, the total time is 10.181, .001 of which it took to perform the first three operations, not .003.