There's only (approximately) two days a month when a human female can get pregnant. It's not about frequency or intensity, rather it's mostly timing. (And sperm quality, but that's another thing people have no clue about.)

Sperm can hang around in the fallopian tube for almost a week. It's also possible to ovulate without a visible period, and it's possible to ovulate in-between "normal" periods.

Don't assume any hard-and-fast rules when it comes to fertility, and definitely don't assume that somebody can only get pregnant two days out of the month. Our species would never have survived if that was really true.

Biology is messy, imprecise, and very subject to external effects.

I was going to say three days, but it doesn’t matter. That’s 7% then turn it into a birthday problem (the world’s full of people having unprotected sex for the first time with someone at any given time) and there you go. Deduct points for bad form or bad sperm and it’s still a crazy number. It’s only a talking point when it’s the case, so it’s bound to sound like it’s happening to everyone.