Quoting Tristan Carman:" I'm right there with you. That's why when I woke up to find my stomach hanging about six inches lower ... [snip!] ... I know I was active and know when the condom broke he's due the first week of November lol. But I use her due date regardless."
Problem is that babies don't always grow at the exact rate they're projected too. Like you may be 20 weeks pregnant but have a baby that's measuring exactly right for 19 weeks because s/he grew a little slower than most at some point. Like with my first the due date they gave me by u/s was 2 weeks later than the one they gave me by my lmp, and by their u/s due date, if my son had grown exactly as per the average, that would mean that I got my bfp before I even conceived (obviously not possible) and yet his little body was exactly on track for their projected due date at every last u/s (all 12 of them). He just grew a little slower than the average, so even if you know exactly when you ovulate or even exactly when you conceived, that doesn't necessarily mean that their projected date is off, it may just mean that your baby grew a little slower at some point.