Weird question?

Lydia
Hi ladies, 
I feel like I should know the answer to this but well, I don't! 
We've been TTC with first baby for 10 months and got our BFP yesterday at 10DPO. 
When I plugged my cycle date into the app, it had me at 3 weeks. How can I be 3 weeks at only 10DPO?
Would appreciate any help on this one!