Glow Accuracy. Confused

I'm a little confused. My AF usually lasts 4 days and I have a average cycle of 36 days. Sometimes a little earlier sometimes later but 36 average. Glow is showing that AF should start on the 22nd which is about 45 days after the start of my last AF. My other apps P-tracker & ff say 35-36 days AF to start on the 13th. Why such a huge variance? P-Tracker is usually spot on when I will get it but hasn't been successful in getting pregnant. I am confused