Some advice please

Annette • Just got my bfp with our rainbow baby 🌈
So glow says my AF is due on 21st June ( this is my first month using glow) my normal cycle length is 31 days. I ovulated on 7th June. I'm confused as my last period was 25th may so if I count from 25 th May to 21st June ( next expected AF ) it's only 28 days! So has glow got my AF wrong as I'm 31 days normally? Any advice would be appreciated. Thanks :)