How is she underweight?

Okay I’m 33 weeks exactly today and had a ultrasound baby weighs 4.4 lbs right now and they doing another ultrasound in two weeks to check the weight again and I had one two weeks ago and she was 3.1 lbs. they saying she’s underweight and should weigh more but all my apps say baby should be about 4.2 lbs this week, wouldn’t that make her just where she should be at right now, what do you all think? She moves all the time and heartbeat always great. How much does your baby weigh at 33 weeks?