That can be because their body did not develop any antibodies yet, can take 15 days - or even worse some people aren't developing antibodies at all according to some studies - which doesn't contradict the 80/90% but it does question the herd immunity...
These kinds of things were exactly why I started this thread (though the title should have probably been better phrased as a question, which is what it was intended to be).
I had certain symptoms and landed in the ER but was never tested for COVID-19 because I didn't have any respiratory issues. I have fully recovered, gave some blood to the Mt Sinai plasma program to be tested for antibodies, but never heard back from them.
I see people who are presumably fully exposed (within same household with no distancing) and don't get any symptoms (and even test negative).
I see older people get very mild symptoms, and young people ending up in the hospital on a ventilator.
Different people get totally different symptoms.
I just can't seem to find a rhyme or reason to all of this in my mind. Which is why I started this thread asking people to post scientific articles (or at least news articles pointing to scientific studies) about the virus, treatments, etc.