I'm also positive that it is not preordered what will happen with certain Sim at certain day etc. Pregnant Sims are created pregnant, have skills and careers, and relationships (actually they are placed in CAW by creators, we can do that only with mods). There is no way EA programmed "the future" of precreated Sims. There is CAS random generator, many names generators, not shore why you doubt it.
I had a controlable pregnant Sim getting twin boys but lost that file, so I upload previous saved file, when she was 2 days pregnant. This time, she had only one son, with completely different genetic and favorites than previously born twins from that same pregnancy. I also did some testing, and it seems that newborn characteristics are determined when(or just after) you get the message to name a boy/girl and chose the traits. So if you save while mother is in hospital, you can test many many times what would baby look like and what are the favs. And when there is so much randomness in controlable household, in uncontrollable are even more.
There are some factors in the randomness which are already mentioned, but they are not exclusive. Family Oriented Sims will most likely have large families, and Ambitious Sims will probably progress faster in a Career, you'll find Angler fishing much more often than other Sims. But I had Christopher Steel being extremely large (looking for a nice word for being extremely fat), and engaged to many many different townies, from Monika Moriss to Yumi Sekemoto - Leighton's mother even if in romantic interest with a Sim I controlled. I also saw a lot of movements from Alto's and Wolf's. Some strange relationship evolve like Mortimer get engaged with "never seen or heard of" Sim, or Sam Sekemoto had 3 wives and 5 children with all of them.
One more thing, Jamie and Thorton are only friends, no romantic interest, and I never saw them flirt, but am shore it can happen as this game is designed to surprise us.