How did I get pregnant

I'm pregnant but the whole time during my pregnancy I've been wondering how just bc statistically its mind boggling. The month I concieved I relied on the pullout method, but my boyfriend said he was doing it properly and at the time we had only been sleeping together two months. So it just seems so unlikely that precum maybe when I ovulated maybe once would get me pregnant. I keep wondering how or If he came in my accidentally and I didn't notice it. In my past relationships when I have been on birth control and let people cum in me I have clearly felt the cum inside me, I'm confused.