Oh, I've read them now. There are some answers about something we used to call "soft real time". Those OSes (Linux was one, for a while, it's not anymore) will have some processes marked as real time, and change the scheduling so that they will execute right at the correct moments. The gotcha is that there can not be many real time processes, since the OS has no way to resolve a conflict between them.

What I described is called "hard real time". It is indeed a very different thing. Both kinds of OSes are in current use, on different niches.