Confused about weeks

I feel like an idiot, got my BFP last night, woohoo! Updated my status in glow and downloaded glow <a href="https://play.google.com/store/apps/details?id=com.glow.android.nurture">nurture</a>.

When I put the date of my last period, it calculated me at 4 weeks. How can I be 4 weeks if I only ovulated 2 weeks ago??