Chances of pregnancy?

So I’m on birth control and my partner and i have sex regularly. I take my birth control every night at the same time. He and i wear condoms but sometimes we don’t and he usually cums in my mouth. When we use a condom he sums in the condom. Don’t worry we get std tested regularly. I just get worried bc i see girls getting pregnant and they say they were on birth control. So how do they get pregnant? Do they not take it regularly?? This one girl said she did and just got pregnant. So I’m confused