Condoms, birth control, and women still get pregnant?

So a lot of women keep talking about how they got pregnant on Nuvaring and it’s making me worry a little bit. I’ve been on it for a month and just started my second ring. I still always use condoms as a backup but I have had condoms break while on the ring. I had my withdrawal bleed so I’m fine. I also read about women getting pregnant from condoms and being on a form of birth control which doesn’t make sense to me at all. If they were on birth control AND they used a condom and it didn’t break then how the heck would they get pregnant