Anyone??

So this is my first time really using a period tracker app. I've found this app to be absolutely incredible so far! Like I'm majorly impressed! But I have a question for people who have used this app for longer than me (this is my like 2nd or 3rd month).

My periods within the past several months have started towards the very end of the month. I logged my June period on here and eve was pretty close to guessing my July start date. But now we're in August and it said I was supposed to start today. I always start like first thing in the morning, and I haven't yet. I'm not concerned about my period being late or anything because I know I'm not pregnant, but is this app super reliable? Like my last period was around the 21 or 22 and now it's saying I should start on the 16... Does that seem right??