Estimated due date predictions on apps

Why do some apps say doctors count from the 1st day of your period and not the last. I was always told it's the last day. Even when I went to urgent care they said it's the last day but I have now 6 pregnancy apps. Just to read articles and see the difference in information given and literally 5 out of the 6 say I'm about 1-2 weeks ahead than what nurture/glow says. I don't know my due date because they want me to wait for a sono due to my irregular periods. But these apps are so confusing....