YouTube 720p audio bitrate re-encoded? (720p 192kbps, but others 126kbps)

-download videos at various quality -> extracted audio streams -> compared bitrates and checksums

-480p and 1080p audio streams have identical checksum (also corroborates with Replay Media Catcher 5), but 720p audio stream did not match.

-Identical results obtained for multiple videos.
-Identical results obtained from test video I created with a 434 kbps audio stream [480p=126 kbps/720p=192 kbps/1080p=126 kbps]

Summary of results:

4K Video Downloader:
480p 126 kbps aac
>>720p 192 kbps aac [re-encoded?]
1080p 126 kbps aac

Applian Replay Media Catcher 5:
480p 126 kbps aac
720p 126 kbps aac
1080p 126 kbps aac