Lap timer not updating best lap milliseconds...not efficient enough?

Hi

I have a lap timer script which currently times the current lap but also stores the players best lap and displays it to them. I store it in playerprefs. the problem I have is that if I go milliseconds faster, the best lap isn't being updated. If I go seconds faster, it is... so 00:09:90 .. if i go 00:09:50 .. it wont update.. as soon as i do a 00:08:++ it will update...

I have included the script here from one of my levels. Hopefully its fairly obvious as to what each bit does... Any ideas?

 #pragma strict
 var myStartTime:float;
 var GameStartTime : float = 0;
 var isStartTime : boolean = false;
 
 var BestTimeText: String;
 var minutes:int=0;
 var seconds:int=0;
 var fraction:int=0;
 var timeText;
 var objecttoshow : GameObject;
 var newSkin : GUISkin;
 
 function Start () {
     if(PlayerPrefs.GetInt("StadiumBestTime") == null || PlayerPrefs.GetInt("StadiumBestTime") == 0){
         PlayerPrefs.SetInt("StadiumBestTime",100);
         PlayerPrefs.Save();
     }
 }
 
 function Update () {
     BestTimeText = String.Format ("{0:00}:{1:00}:{2:00}",PlayerPrefs.GetInt("Stadiumminutes"), PlayerPrefs.GetInt("Stadiumseconds"), PlayerPrefs.GetInt("Stadiumfraction") );
     
 }
 
 function OnGUI() {
     GUI.skin = newSkin;
     GUI.Label (new Rect(Screen.width/2-Screen.width/4,Screen.height/3,Screen.width/2,Screen.width/12),"Best Time: "+BestTimeText);
 
     if(isStartTime)
         GameStartTime =Mathf.Abs( Time.time - myStartTime );
     else
         GameStartTime = 0;
     minutes =GameStartTime / 60f;
     seconds = GameStartTime % 60f;
     fraction = (GameStartTime * 100) % 100;
     timeText = String.Format ("{0:00}:{1:00}:{2:00}",minutes,seconds,fraction);
     GUI.Label (new Rect(Screen.width/2-Screen.width/4,0,Screen.width/2,Screen.width/12),"Time: "+timeText);
     
 }
 
 function OnTriggerEnter (collider : Collider) {
 
     if(collider.tag=="TimeEnd"){
         if(GameStartTime < PlayerPrefs.GetInt("StadiumBestTime")) {
             PlayerPrefs.SetInt("Stadiumminutes",minutes);
             PlayerPrefs.SetInt("Stadiumseconds",seconds);
             PlayerPrefs.SetInt("Stadiumfraction",fraction);
             PlayerPrefs.SetInt("StadiumBestTime",GameStartTime);
             PlayerPrefs.Save();
         }
         Time.timeScale =1;
     }
     
     if(collider.tag == "start") {
         myStartTime = Time.time;
         isStartTime = true;
         yield new WaitForSeconds(0.5);
         objecttoshow.SetActive(false);
     }
     if(collider.tag == "mid") {
         yield new WaitForSeconds(1.5);
         objecttoshow.SetActive(true);
         }
 }