It really does seem like some people are just unlucky and end up with tooth decay despite fastidious brushing and flossing habits, while other more fortunate souls have never purchased floss and don't always remember to brush but have perfect teeth for their entire lives. There are a lot of hypotheses surrounding the role of oral bacteria in tooth decay, but as it turns out, the explanations that have been widely accepted for decades might not be the
whole story. It's like there's some unidentified factor that disrupts the oral microbiome and creates the right environment for cariogenic bacteria to flourish. It's actually kind of fascinating. I would not be surprised if it were the kind of process that's very difficult to disrupt once it's set in motion, so once you get a cavity or two, you're more likely to continue to get them even in other teeth.
I don't know that having ten cavities at every checkup is attributable to mere bad luck, though. Assuming she's going to the dentist at least annually, that's a lot of decay in a short time. That it's happened to her twice in a relatively short period is also interesting.
My own crackpot theory: those shitty little floss picks don't do the job as well as actual floss. In addition to the inferior mechanics, it seems like jamming the same manky bit of floss between all your teeth over and over again just serves to spread the filth around. It's lazy as fuck. If you have two working hands, there's no excuse for not using actual floss.