Opinions please!

I'm so confused. I've been using 2 different apps to track my ovulation. Glow always says I'm ovulating one week before the other app says I'm supposed to be ovulating. I'm confused on which one to go by and why would they be so different? I use ovulation tests and still haven't been able to pin point my exact ovulation day. I'm just wondering how 2 apps can say completely different things. I mean a few days difference I can understand, but a whole week! Does anyone know which is more accurate?