Slynd efficacy question?

I’ve been on slynd for a month and a half and I love it, barely any side effects. I am terrified about its efficacy though. European trials showed that it was over 99%, which is what I was banking on, but US trials showed like 96-98%. And the website says 96% which nurses claim is with perfect use. This is just not enough for me to feel safe with. Can anyone explain why these trials are different and if the lower numbers are correct, why this pill is less effective than others?