App difference with dr counts

hi mummies , I would like to know if there is a difference of dr counting weeks of pregnancy and the app is usually different ? If as follow what my dr say , I will be at 34 weeks tmrw however my app shows me that I will be 35 weeks tomorrow. And my due date by app is 18 Dec instead of 24 Dec as mention by the dr . The app count by my last first day of period.