I've tried to Google this and have got nothing. I understand that birth control stops ovulation, which keeps you from getting pregnant. So if a woman is on Nuva Ring. And she puts it in on the first day of her cycle, keeps it in for three weeks, then takes it out and puts a new one in a week later. And stores them at the correct temperature and does EVERYTHING as directed and NEVER misses a day or messes anything up and always uses condoms during intercourse... then how can she possibly get pregnant?

Because birth control isn't a 100%
Most are only like 98% but it probably differs from brand to brand.

Agree, it does not stop it completely, There are people, a few here on this site as experts who were on two types, and still got pregnant.

There is no 100 percent method. Even best may say ( and I don't believe them) that they are 99.99 percent.
But that means out of 10,000 times, one time could get you pregnant.

But many rate theirself as 95 to98 percent.

Condoms ( while not mentioned) are far less, some studies lower than 90 percent.

So there is no 100 percent , but some methods are better than others