How does the wheel at obgyn tell how far along you are??

Pretty confused according to all my apps, calculators and last period was March 29 that would make me 5 weeks 2 days today. Went in for first appointment and she asked lmp and used some wheel thing and told me im 5 weeks 5 days?? Wth how can that be? Also know that I ovulated April 15 (they didn't ask about that) I guess when they actually measure at a future appt maybe I won't be that far along? Thoughts?