Why Glow?

I use two apps for my period, glow and another. The other app is always spot on for my cycles. O day is right and P-day. Glow is ALWAYS WRONG. Every single month. I enter the same data into both and glow never gets it right. I have been TTC for five years and log all of the same data into BOTH apps and glow is always wrong. Anyone else have this issue?