Glow VS Ovia

I have both apps and they are so different! My AF days on Glow show they are to start on friday the 14th, on Ovia, they are showing as starting on tuesday the 18th. Whats up with that, thats a huge difference in days, how can the accuracy of them possibly be gauged with such a huge difference? I know the days can be changed if needed, but still. Anyone else have this problem?