Figuring out the # of weeks pregnant

Amber • Married💍 Nolan James 👶🏼


So i recently found out I was pregnant and when I downloaded this app and put the date of my last period it said I was 2 weeks and 6 days with a DD in late Jan. I just switched my glow account to pregnant and it said I am four weeks pregnant with a DD of Jan. 15? If they are both using the same date of my last period how could they be different?