Glow ovulation vs OPK

I've been TTC for 1.5 years now and know more than I should about fertility, haha. I just started using the glow app because my old app has started not functioning properly. This morning I got my positive opk, which means in 24-36 hours I'll be ovulating. Why is it that the app says I have the highest chance of getting pregnant today? It should be tomorrow. Any ideas? As a background, I'm on my second round of IUI, and my doc has me call in when I get a positive opk, and schedule the IUI for the NEXT day (ie: tomorrow).