Encoded AAC is always shorter than Video

[mars2010]

Hi,
I have a problem in audio file playtime.

I've encoded audio as AAC format,
but it has always shorter playtime than video, H.264.

The expected playtime is 50m, and so is the video.
but the playtime of audio is about 10 secs shorter than it.

So I have a out-of-sync problem by the audio.

Is it possible to face this kind of problem
even though I set the audio bitrate as constant bitrate?

Thanks in advance for your help.