I can't get accurate playheadTime in flvplayback component.

I use flvplayback to play a 20 seconds flv video, but I can't get accurate playheadTime no matter how to set the attributes.

Here is my as3 codes.

import mx.video.*;
my_FLVPlybk.playheadUpdateInterval=10;
//my_FLVPlybk.seekBarInterval=100;
//FLVPlayback.seekBarScrubTolerance=100;
function the_play(eventObject:Object)
{
ta.text = my_FLVPlybk.playheadTime;     //  ta is a textarea.
}
function the_end(eventObject:Object)
{
ta.text = "video over";
}
my_FLVPlybk.addEventListener("playheadUpdate", the_play);
my_FLVPlybk.addEventListener("complete", the_end);

I drag seekbar to between 0 and 8 seconds, it always obliges to 8.333 .