Low predicted birthweight or normal? Doctors wrong?

I am currently 24 weeks and at 21 weeks the Dr did an ultrasound and found that the baby's weight was in the 17th percentile. He says the baby has been growing in the same curve so she has not stopped growing or anything, she just seems to be smaller than average. But, my question is before I got pregnant I was charting my ovulation and so im pretty sure when I got pregnant and it coincides with the first date they gave me which was dec 30. Afterward they changed it to dec 26. Could the diff in those days be what is causing her to be smaller? They say she measures a week younger but that she has been that way since the beginning. I know its only 4 days difference but that combined with the fact that me and my husband are small in stature could mean the diff between a healthy baby and soemthing being wrong. I heard if she drops below 10 percentile that this is abnormal. What does that mean exactly? Thanks so much for your help.