Glow question

I have been ttc for just under 3 years now and in all this time I have been using ovia, it's normally a good few days off with predictions but as I'm quite irregular that's expected. I thought if I get another app and compare both I might have more of an idea when Af is due. This is my first month using glow and I have gone passed my predicted Af But instead of disappearing like ovia does its moved to a week later. Do any of you ladies know if this is what normally happens?