glow predict AF wrong?

CnGd • TTC #1, Stepmomma of 2 beautiful girls 👨‍👩‍👧‍👧
So I have my settings on 31 day cycles. Which I'm pretty regular. Why does glow say my expected period doesn't start till Tuesday? While my period just started today (guess what, today is cycle day 31, like it's supposed to..) 
Oh yeah and that also means I'm out for this month unfortunately.