Ultrasound sizes all over the place?

Amy
At 33 weeks I had an ultra sound done with a specialist. Baby was measuring big...in the 72%. The dr even told me I should try to avoid carbs and sugar so baby doesn't get too big. Had an ultra sound at my dr office today, at 36w 4 days and baby is measuring small...only 29%. I think they said his estimated weight was less than it was at 33 weeks. Anyone else experience this? Are they just pulling these numbers out of thin air, or is it possible the baby would have stopped growing/gotten smaller. I am so confused.