Help understanding bbt chart.

This is my first month using the chart feature and I'm still trying to understand everything that goes into ttc. I've noticed that my chart had a fairly significant dip then rise glow told me this is a sign of ovulation but I ovulated on the 15th this was confirmed by an opk, thee dip I'm referring to is on the 20th. I haven't been doing anything different my house is the same temp always. I would love some insight as to what this could mean?