Encoding wav files to ac3 creates files of different time length

[Amarena21]

Hi everyone, I'm having troubles encoding wav files to ac3.

My current workflow is untouched ac3 -> wav (i convert to wave to time stretch the audio) -> ac3.

Unfortunately once I finish time stretching these wav files and I export them to ac3 with the built-in ac3 encoder in Adobe Audition CC 2017 the length is always a little longer compared to the original wav files.

I also tried reencoding these wav files with the ffmpeg encoder (with Audacity) and also useac3to gui hoping that maybe using these other tools the length would remain unchanged but it doesn't.

The amount of milliseconds added in the ac3 encodes varies depending on the wav file and it usually goes from 20 to 50 milliseconds, but it remains the same no matter which encoder I'm using (Adobe Audition, ffmpeg library (with Audacity) or useac3to) leading me to think that this might be an issue with the ac3 codec itself not with a speciifc encoder.

Does anyone know a way to make sure that the length of an audio track remains the same once converted to ac3?

Thanks to everyone who can help.