due dates

Okay so I'm pretty confused on how <a href="https://play.google.com/store/apps/details?id=com.glow.android.nurture">nurture</a> estimates due dates. It asked me the first day of my last menstrual period and I entered it in. It then told me I'm 3 weeks and 5 days but I'm only 11 dpo. Is this true?? Or would my due date be later. I'm going to the doctor next week to get a better estimate but I'm just curious