You had a scan done at the hospital and the sono said you were 6 weeks pregnant but then you went to your doc and he said you were 4-5 weeks y would this be?

Doctors are scientists, not psychics. Your doctor was making his best guess with what information he had. Chances are the scan provided more detailed information and is more correct. The exact date of conception is nearly impossible to figure out unless the woman can say with certainty.