This was streaming to a remote chromecast which had bandwidth limited to 3 Mbps.
The streaming brain doesn´t always produce the wisest decision for the transcoding used, i´ve seen this on so many occasions now.
You are right, but that doesn´t mean there is no room for optimization. The transcoder decided to fiddle with both, audio and video, whereas it could just transcode audio and get a good result, or even crank up the bitrate a little more on the audio side an still be within the 3Mbps limits.
Watching transcoded material is slightly less entertaining, as you need to wait for the transcoder before the movie starts und the same for skipping around.
Maybe the transcoder could check if transcoding audio (with the same number of channels) is already enough to be within limits and direct stream the video part. Would also reduce the load on the server, which is always welcome.