Bbt & opk + dont match?

This is my first month charting, so maybe I'm reading these wrong, but it seems like going off of my bbt that ovulation was CD18 on 3/22, but according to my opk, my lh peak was around 3/19 or 3/20, which (if I remember correctly ovulation being 12-24 hours following surge), puts ovulation at CD20 or CD21 (3/24 or 3/25)... is there usually a 2-3 day variance? I wasnt expecting that.

(Also due to personal reasons we decided on 3/22 not to actually try this month)