FlashBuilder generated swf files are different

I am using FlashBuilder to create flash file to show streaming video.  Please see the following code:

<?xml version="1.0" encoding="utf-8"?>

<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009"

               xmlns:s="library://ns.adobe.com/flex/spark"

               xmlns:mx="library://ns.adobe.com/flex/mx" minWidth="1280" minHeight="720">

    <fx:Declarations>

        <!-- Place non-visual elements here, for example, services and value objects -->

    </fx:Declarations>

    <fx:Script>

        <![CDATA[

            import mx.events.FlexEvent;

            import org.osmf.net.StreamingURLResource; import org.osmf.net.FMSURL;

            protected function vp_preinitializeHandler(event:FlexEvent): void

            {

                var myURL:StreamingURLResource = new StreamingURLResource("http://www.sample-videos.com/video/mp4/720/big_buck_bunny_720p_1mb.mp4");

                myURL.urlIncludesFMSApplicationInstance = true;

                myVideoPlayer.source = myURL;

            }

        ]]>

    </fx:Script>

    <s:VideoPlayer id="myVideoPlayer" autoPlay="true" preinitialize="vp_preinitializeHandler(event)"  x="2" y="2"  height="100%" width="100%" />

</s:Application>

After I click Menu >> Project >> "Export Release Build", FlashBuilder generates a swf file, which looks fine.  However, I found that the swf file changed EVEN IF I did not change any code.  In other words, the same code generates different swf files in each "Export Release Build" click.  I used command line fc to compare the generated swf files and confirmed each file is different.

I installed FlashBuilder of the newest version (ver 4.7 from Adobe). I wonder why the same code generates different swf files, and what kind of information is included in those different swf files.

Your help is greatly appreciated.