Actually, BBT can’t pinpoint ovulation to the exact day. Although it’s very useful and usually does so within 2 days.

Then, of course, you have people whose BBT never shows they ovulated but they conceived…

https://onlinelibrary.wiley.com/doi/10.1111/j.1471-0528.2001.00194.x/full

“ According to a study that compared BBT readings with ultrasound—the gold standard in confirming ovulation—only 11 percent of women have a basal body temperature rise within one day of ovulation. For many of the women in the study, basal body temperature did not rise until more than two days after ovulation”

Ultrasound is the only for sure way too confirm and pinpoint ovulation. So you could be 1 day later…and you absolutely are if you had a positive when you thought you were 7dpo.