Ovulation

Lanie • 21 yrs old. Happily engaged. Mother to a beautiful baby girl🎀
According to glow a few days ago it said I was supposed to start ovulating on the 17th, but I noticed today it changed to the 21st. Why did it change? How does glow calculate when you're supposed to be ovulating? I don't record my temperature everyday, I hope it's not off that ?