It's basically completely random which sperm fertilizes any given egg. If you had to roll for it on dice, you'd need big dice and a very, very long time to reroll them. Why should the dice rolls stay the same when you're rewriting everything else from the top? Follow this logic to the conclusion, and sooner or later you're going to wind up with this trope in force: Original Characters Only.
People who were butterflied away by events that didn't happen IOTL will obviously be having fewer kids. People who survived because IOTL they died from things that were butterflied away will obviously be having more, and those children will be original characters. As the number of Original Characters rises and the Historic Characters falls, it's increasingly likely that more of the real people will marry with original ones instead of their historic counterparts. All signs indicate that this trope is inevitable.
For all that, this is a trope that's very rarely invoked, for a few relatively simple reasons:
- Creating people is hard work. Not just "names and personalities" hard, either.
- It's really uncanny.