Manual captioning is almost always not done by domain experts, but by people who have some training with a captioning system and work as professional captionists. Their main advantage is that they'll caption much faster and much cheaper than having domain experts do it, but the quality tends to suffer.

In college, I met a deaf guy who always had two women accompany him to lectures; one of them would repeat everything into a mouth-covering microphone to generate an automatic transcription and the other went over it to correct obvious errors. They generated a lot of nonsense, especially when the German professor was using some English loanwords for CS concepts. I was always amazed that the deaf guy still somehow managed to learn something from these garbled transcriptions.