It might be pulling an audio format that either youtube resampled, or that ffmpeg had to resample when merging the video because it's not compatible with the video format.
There's a way to pick preferred audio and video formats it will choose over what's simply the best bitrate, but you'll have to look in the manual for more details.
There's a way to pick preferred audio and video formats it will choose over what's simply the best bitrate, but you'll have to look in the manual for more details.