Why are bytes growing in the AS3 preloader?

I am creating a custom preloader for a Flex application and noticed the following behavior: on load, the progress bar goes to 100%, then down, then back up, etc. until the app finishes loading.

When I put a trace in the dowloadprogress listener, I see that while the app is loading, both the loaded bytes and bytesTotal are incremented, but not necessarily at the same time.

the code:

private function onDownloadProgress(event:ProgressEvent):void {
        var loaded:int = event.bytesLoaded;
        var total:int = event.bytesTotal;
        trace(event.target,loaded,total);
        _starfield.progress = loaded/total;
    }

      

        
        
        
      

    

Output:

[object Preloader] 134276 134276
[object Preloader] 265348 285007
[object Preloader] 285007 285007
[object Preloader] 678223 1322116
[object Preloader] 809295 1322116
[object Preloader] 1322116 1322116
[object Preloader] 1322116 1322116
[object Preloader] 1387652 1584342
[object Preloader] 1791882 1791882
[object Preloader] 2293133 2293133
[object Preloader] 2362938 2362938
[object Preloader] 2362938 2362938
[object Preloader] 2362938 2362938

      

        
        
        
      

    

Why do the bytes change when loaded?