How accurate are fundal height measurements...

At our 20 week anatomy scan baby was about 2 weeks ahead, and I’m huge so I was expecting a fundal measurement ahead. But was pretty shocked today at my appointment. I’m currently 24 weeks and expected to measure about 26-27. Doc said I’m 30cm. 😳 how accurate were your measurements? He said he knew baby was going to measure big from the ultrasound and that if I’m till big next time (4weeks) we will do an ultrasound. Were your measurements big for fundal height and accurate? Or did they even out. I feel like a few weeks ahead is normal but 6 is kinda extreme...