confused about peak fertility day

I think the app should take into account what type of conception is planned. I am doing an IUI where timing is very important and IUI should be done the closest to ovulation, usually 24 hrs after positive opk. I also know my luteal phase length (13 days). I am very regular and due to ovulate June 2 and planning IUI for that morning.

But the app is giving me saturday May 31 as top fertility (28%) and only 6% for June 2. even though that is the closest to ovulation. I'm confused..