I have noticed that, too. For taped and filmed stuff I think it is done while it is being recorded. TV stations are required by the FCC to provide closed captioning even for live stuff - which is really difficult. I asked him how it is done and this was his reply:
"We type the stuff for the news (it’s actually the feed from the teleprompter being sent to the closed captioning system)..
I think for network shows they have someone who transcribes them while they’re being taped.
For live shows, they have someone who can type really fast doing it live. There’s a lady in Austin, TX who does that for a living…she just listens to the show over a phone line and types into some internet connection that’s piped back to the network in NY."
So I guess that would explain spelling errors.