When does it mean when a doctor considers the lmp due date and conception if the ultrasound is in a 7-10 day agreement with the lmp? need an example

Fetal dating: 1st trimester ultrasound has margin of error of 5-7 days for due date. If due date by LMP is 9/22/13 and us due date is 9/27/13, they are in agreement, and typically LMP is used. However if ultrasound due date is different by over 7 days, i.e. 10/8/13, then the ultrasound dating would be used. The ultrasound can be off by up to 2 weeks by second trimester, and 3 weeks by 3rd.