Differences between ovia and glow

Summer • Age 41, ttc for four years, one chemical pregnancy, praying for just one baby!

Ok so I entered the exact same af info into the ovia ap that I entered into glow. Ovia is predicting af in three days (and I have no symptoms right now). Glow is predicting in five days. My cycle is generally 28 days. First day of my last period was July 11. Curious how in the world these predictions are so different with the exact same info entered. Anyone else notice this with two or more apps? I just want to know better when to test!!!