what the world?

Katlyne β€’ Single Momma of 2πŸ’œ Annabella Desire’e Lee πŸ‘ΆπŸ»πŸ‘‘πŸ‘ΈπŸ»πŸŒˆ 12.04.17, 8lbs 5.5ozs. Edward Alekzander πŸ‘ΆπŸ»πŸ‘‘πŸ€΄πŸ»β™₯️ 02.06.19, 8lbs 12ozs.
So call me crazy, but I've got 7 apps to track my period and fertile/ovulation days. 1 of the apps says I became fertile on the 4th. 4 of the apps say I became fertile on the 5th. 1 app says I became fertile on the 6th. Then there's glow who says I become fertile on the 10th..... all the other ones are a lot closer together than the 10th lol. And I have a 31 day cycle. Which means I normally ovulate between day 14-17. So I'm just trying to figure out why glow is so off... any ideas?Β 
(Yes I know they're just estimates of everything, I'm just trying to see if anyone else has other apps that are close estimates then glow being far off)Β