Ultrasound measurements

I am currently 19 weeks and a few days. Yesterday, at my ultrasound they said the baby's body and various body parts were measuring 19 weeks and 3 or 4 days (depending on the different parts they were talking about), but when they did the head they said 18 weeks and like 5 days. I know ultrasound measurements can be off, but is it weird that the head is measuring as less developed than the rest of the body? Has anyone had that happen? The tech couldn't say anything, because she isn't the doctor, and my appointment is a week away. She did say they weren't far off, but it seemed weird.