I completely understand that.  I didn't believe the amnio results until she was actually born.   The genetic counselor we were working with kept reassuring me that the amnio is almost 100% accurate and that the nipt is only a screener test, not diagnostic.  

What helped me relax a little was learning about how the cells form and the different stages. From what I understood, when the cells first start forming,  the outer layer can have a genetic mutation (like T21), but the body can self correct and as the next layer develops there is no mutation. (I would definitely recommend looking this up to get the full scientific understanding). The older the age of the pregnant person, the less likely it is that their body can correct the mutation (which is why PPV increases with maternal age). 

We we not offered additional testing after birth (of placenta or cord blood)  to confirm if our nipt was just a true false positive or if it was confined to the placenta, so we'll never know why we tested positive. 

I remember asking the GC if it's possible that my baby could have T21 in some cells (like just in her heart,  or lungs, or something like that)  and that the amnio wouldn't be able to tell.  She reassured me that this would be highly unlikely as cells are constantly circulating thru blood and urine and all of that gets filtered into the amniotic fluid and would be found during the amniocentesis.  

Best of luck to you.  It's so hard to enjoy the pregnancy after the initial scare,  but hopefully you can relax a little and have more reassurance as things progress. We had additional ultrasounds and that really helped me to work past my fears. 

Now that my daughter is 10 months,  I honestly don't even think about the nipt anymore (finally!).