4 apps 2 different results

Kimberley
Hey ladies. I have 4 period and <a href="https://play.google.com/store/apps/details?id=com.glow.android">ovulation apps</a>. All the same info but 2 say I ovulate one day and the other 2 day I ovulate the day after! It's weird. Anyone know why and is it best just to get the ovulation kits?