Different ovulation apps

So I just recently started using glow and before that I was just using a plain Jane app for when my next period would be and it also gave when I would be in my "green week." Well, my glow says I'll be ovulating this week, but the other app says next week.. It might be a stupid question to ask, but I just didn't know if anybody knew why this was? I took an ovulation test tonight and it didn't say I was getting ready to ovulate, but was gonna try again in the morning.. Thoughts???