If it was a sixth-week or seventh-week ultrasound, the projections from it could possibly be spot-on, but even by
the tenth week
babies begin to develop at different rates. By week 12, it can be +/- 7 days either direction.
Correct me if I am wrong about your dates --
September 21, date projected by an ultrasound to be your conception date
September 23, sex with guy #1
September 27, date projected by 10w4d ultrasound as conception date
September 28, sex with guy #2
Is this right?