Did I ovulate cd 19 or 20?

Em • 1 Samuel 1:27 | EDD 11.21.17 💚
Hi. I'm trying to learn more about charting. I'm out for this month because my temp dropped today, but does anyone know why CD 20 is indicated as O day instead of CD 19 (the lowest temp of the 2 and the date that makes more sense looking at cm)? I'm trying to learn more about charting so if someone could explain I'd really appreciate it. Thank you all very much. 
(Oh and I think my crosshairs are dotted because this was my first month tempting. The app says they need to see your cycle for two months to really know how your body works.)