Nothing to stress about!

Ultrasound measurements are always +/- seven days. They won't even consider moving your EDD unless your consistently getting measurements that are more than 7 days out either way over a couple of scans.

Because all babies grow at slightly different rates it is not, and will never be an exact science. As jgal said the first 12 weeks they grow at pretty much the same rate (which is why early pregnancy scans are the most reliable for dating). But as the pregnancy progresses genetic factors also begin to come into play - eg. baby might be measuring smaller because one or both parents are smaller etc so the measurements begin to get less reliable until by the time you're full term you might as well flip a coin as rely on an ultrasound weight estimate. The skill of the technician also becomes more of a factor as well. This is partly the reason that so many mothers are given estimates of full term weight that are notoriously inaccurate (I was at a birth on the weekend where the mum had been told a week ago that she was having an 8lb baby - baby came out at over 9 pounds ).

It sounds like everything is fine, don't worry.