Those are estimated dates. It could be up to 7 days before or after that date depending on when your body ovulates.
My tracker says I conceived on july 31st but i know i conceived after august 4th. Some people ovulate a little later than the norm :)
Trackers arent always 100% everyone ovulates differently, maybe your cycle isn't exactly as on track as you thought. Anyways, congrats, when you go to your first prenatal appt they usually have ways to guess your due date! Welcone to the forum!!!!