The measurements that they also use to calculate babies weight in utero is their femur measurement, so for all you know, you could have a really long, 8lb baby kwim?
It depends on how detailed they actually do the growth scan, I had a growth scan on DD as they were checking her size due to excess amniotic fluid and were check for GD as well, so they checked her growth to see if she was bigger than the average. To do it hey took at lot of measurements, HC, abdomen, limb length, like the femur, she also measure the amount of amniotic fluid and all the figures went together. And a figure was generated based on that, which I believe is significantly more accurate then just using the femur measurement. The people I know who had had the same level of scan as I did, that it was significantly more accurate than just the femur length, but by no means is it failsafe.

In reality they are attempting to measure a 3 dimensional bubba in two dimensions so certainly room for error