Two different answers

I am currently using two different apps to track my cycle. I have input the same information into both however each one has separate answers for fertile days and expected days of AF. <a href="">Glow app</a> is three days a head of the other app. For example, glow states AF should come 1/1/14 - 28 day cycle. The other app states AF due on 1/4/14 - 31 day cycle. I am not sure why the differences. Anyone else have this issue?