glow is confusing me!

Last month I recorded my positive ovulation on day 15 and it put ovulation at day 16. My period came day 29 so I obviously ovulated at day 15. This month it is doing the same I got my peak today which is day 15, and it's predicting ovulation tomorrow, I'm sure I already released an egg and I can tell by the pains, and my period is predicted again on day 30. Why? Every month glow is a day out. Grr!