why did MediaRecorder delay to record audio

I use a MediaRecorder to start record audio, and at the same start a TimerTask scheduleAtFixedRate(doRecordingTimer, 0, 1000); and a Handler to update UI showing the time has passed.

Here comes the problem: When the UI shows 10 seconds(just for an example) have passed, and I stop recording, start playing the file, but it just shows as a 4 seconds long audio file.

Why does this开发者_如何学C happen? I can be sure the time on the UI is correct, so what cause the MediaRecorder to delay?