Baby growing

Had my 20 week ultrasound the other day and they told me my baby is growing 9 days ahead. They kept questioning me if I took a guess of my last cycle and I told them no if I am off it would only be 2 days. Anyways my due date is still the same because they can't change it unless it's a 11 day difference. My question is if anyone experienced this or if their baby is growing a tad faster than average?