Usually the doctor or technician puts the LMP date in. The machine generally only calculates a gestational age based on certain measurements.
The tech puts in the info for lmp, the machine will calculate gestational age based on crown to rump length measurements for early pregnancy or for later pregnancy a combination of femur length, abdominal circumference, head circumference and a few other determining factors. I would mention it to the doc and see what s/he says.