Getting so frustrated

I have 3 apps total(including glow). One app says my fertile window is 11 days long, starting after my period, ovulation day the 8th. Then another app says my fertile window is 6 days long with ovulation on the 9th. Then glow says my fertile window started on the 9th, with ovulation happening on the 14th(or 15th, one of those days. I can't look bc I am writing this). It's all driving me crazy with all these different days. Basically I'm "fertile" the whole month except for my period week. I really don't understand it at all, how do they come up with these numbers?