Confused by app

Ok so AF was supposed to come yesterday(Christmas) according to the apps calculations dating from my last MC (which I know is the correct start date bc I downloaded the app that day. Moving forward, today(12.26) I logged in and clicked "period late" and that's where I got confused because now it is saying my next period is Monday 12.28. However, my period has been starting late lately(last 30 days).