Duration of M4A file is wrong?

[d4eva]

I think duration is an issue after all.
I attached files to demonstrate it.

8.wav and 9.wav are original ones.
There should not be any gap in between (those are small pieces of tracks 8 and 9 from Depeche Mode Violator).

When I concatenate files 8.wav and 9.wav and then open resulting 10w.wav in Audacity I see that there's no gap in between, so this is OK. It sound fine as well.

ffmpeg -f concat -i ccw.txt  10w.wav

8.m4a and 9.m4a are ones created with FFmpeg.
When I concatenate them like this and then open resulting 10.wav in Audacity I see that there is a gap in between. That can be heard as well.

ffmpeg -f concat -i cc.txt  10.wav