As others have said try not to worry.
With my DD I was given a weight estimate at 34 weeks which did turn out to be right.
With this bub he was measured on the 95th percentile for all his major measurements at my 20 weeks scan and OB was saying he would be 9-9.5lbs.
I had another measurement scan last week and now he thinks about 8lbs.
He was either having a growth spurt at 20 weeks when he was measured or they have no idea LOL.