I was taking my temperature so know exactly when ovulation was and felt a pinching pain which I think was baby implanting but still got three different dates Lauren! If you've had sex more than once leading up to ovulation then there's really no way of knowing precisely.
The thing the midwife uses and calculators aren't accurate as everyone has different cycle lengths and they don't allow for that (although some cycle lengths you can alter on calculators but you have to know your cycles/ovulation dates realy well and for a few months to know properly).
Babies grow pretty much the same up until 12 weeks so that's why they used your dating scan date, but if you had that later, I think you mentioned, then it may be a little less accurate.
Then even with dates, baby still arrives when it's ready and whenever he or she feels like it! x