Ovulation calendar

So glow shows my fertile week to start around the 24th and predicts my ovulation to happen on CD 22, my babymed app says my fertile week will start tomorrow and should ovulate on CD 15. Has anyone had a discrepency like this? I just started using an OPK last month so we'll when it actually does happen but it's just kind of weird to me that they are both so different based off the same info!