No EDD given based on cycle

I didnt know when my last period was so I wasnt given a normal EDD I was given a dating scan at 9 weeks which worked out perfectly as counting back I had sex with my boyfriend 1-2 days before.

My concern is I've heard rumours flying around that even scans are based on average cycles? But seeing as my cycle was unknown mine was based solely on measuremnts of sac and foetus ect. They say you are not truly pregnant until the egg is embedded in the sac which I understand as sometimes unfortunatly in such an early stage problems can occour. But my question is this: having had 3 scans altogether all dating back to the same day (1-2 days after having sex with my boyfriend) surely they based my pregnancy on solely the averages in feotal development so size from crown to rump etc. and that counting back on the scan takes me to the day the egg was fertilized and began developing? Not to a ruff estimate of when it became embedded into the womb?