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.

The highest quality audio is generally tag 251 (webm/opus). You can stream or download through an Invidious instance:

https://vid.puffyan.us/latest_version?itag=251&id=Ry4TqqM5TG...