What Causes Difference In Gestational Age And Actual Date Of Conception?

Hi I was hoping you can help me try and figure this out...I had sex nov 15 w one man and nov 18 thru the 20th multiple times w my kids father and now I am pregnant and I m not 100% sure who s the dad...I don t remember when my lmp was but my dr says nov 20 was my conceive date and on my ultrasound said either the 22 or 23 of November. ..how accurate are those dates and what do you think