HOW did I get pregnant off of precum???

I already have one baby, so I know what it feels like and the after effect when a guy cums in me. Me and my current boyfriend decided we were gonna use condoms and the pullout on my supposed not fertile days. Not once did he ever cum inside of me. He always pulled out minutes before. And we only had sex a total of 3 times before I got pregnant. I will say this. He does watch a lot of porn.... the only thing I can think that happened is he jacked off before me and him had sex and there was still left over sperm in there. I’ve heard getting pregnant off of precum alone is a very small chance so what do y’all think happened?