Ultrasound weight vs actual weight and weight gain

K • After 3.5 years, 6 IUIs, 2 rounds of IVF, & a ruptured ectopic among other things we are FINALLY pregnant. ❤️💜💛💚💙
With my first daughter I had an ultrasound at 35 + 3 and they estimated she weighed 5lbs 13 oz and then at 40 + 4 and they estimated 7lbs 15oz and she was born at 41 + 2 at 6lbs 15.8oz (7lbs) so they were off. 
This go I had an ultrasound at 36 + 3 and they estimated she was 6lbs 14oz 😬. I am guessing I will go late again. I obviously know they can be off but I’m just wondering everyone’s experiences?? Also did how much you gained play any part in it? I know this doesn’t always matter either but I have gained less weight this pregnancy too (so far, 8 pounds less). Just wondering others’ experiences!