I have a script that with the use of a GUIText object, displays the time left before the game is over. The time begins from 15 and reduces. If it reaches 0... game over. I have a question regarding this script:

GUITextTimeLeftEM.js

private var startTime;
static var timeLeftDisplayed : int;
private var timeLeft: float= 15;
var textToShow: GUIText;

function Awake() {
    startTime = Time.time+0.5;
}

function OnGUI () {
    var guiTime = Time.time - startTime;
    timeLeftDisplayed = timeLeft - (guiTime);
    if (timeLeftDisplayed == 0) {
        print ("Time is Over");
        LoadNextLevel ();
    }
}

function Update(){
    textToShow.text = "Time left:   " + timeLeftDisplayed.ToString();
}

function LoadNextLevel () {
        cubeMovementIVEndlessMode.chainHits =0;
        cubeMovementIVEndlessMode.frenzyOn=false;
        yield WaitForSeconds (.5);
        Application.LoadLevel(Application.loadedLevel + 1);
    }

I use

startTime = Time.time+0.5; 

because if I just use:

startTime = Time.time;

the timer goes from 15 to 14 in 0.5" instead of the expected 1". Why is that?