Why are a the ovulation predictions different?!

A. ❤️
I don't know what to trust...haven't tried using opks yet but might have to if it keeps confusing me... 
Glow says fertile period was 9/7-9/13 
​Ovia says 9/9-9/14
​MyDays says 9/4-9/9 (!)
​I don't know who to go by or even what they're basing that on!