Does Glow get it wrong?

Mary • 20 years old, engaged, mommy to a babygirl born on July 22nd, fur momma to my Yorkie-Poo, Zelda :)
Hey everyone! 
 So, for the past couple of months, I have noticed that Glow predicts my period to arrive at a certain time and most of the time it doesn't show during that time. So, according to that some of my periods have been "late". The other <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">period app</a> I use (Ovia) usually says something completely different from what the <a href="https://play.google.com/store/apps/details?id=com.glow.android">glow app</a> says. So, my question is, which of these two apps are right or wrong? Does this ever happen to you? I fill them out one right after the other so I remember everything.