Hmm I'm not sure, that's never happened to me! Maybe your surge started later in the day on cd17 after you did your opk and you ovulated quickly after it that night. The temps really suggest that day for O but it could be the next days too. I'm pretty sure this cycle I had my surge around 1 pm then I could feel O pains in the evening about 8 pm just as an example. But your bd timing looks great for any of the days so maybe just keep it in your mind that you may have a range of dates for O, that way you don't stress out as much thinking you're farther along when it's testing time. Also as you get more temps logged into your chart, a more obvious pattern may show up. Maybe your temp will spike up higher in the next day or two and you'll be able to see it better. Best of luck!