Apps off on due date?

Stephanie • Mom of three beautiful babies, jaden 10, alivea 7, and averie 4. My fiance and i are TTC baby #4!
How can two different apps be a week off with my due date? I know the exact day I ovulated and kept track of cm and bbt to confirm. I'm supposed to be 4 weeks today but the other app says 3 weeks today. I don't get it.