low hcg based on LMP?

my LMP was 5/31. cycles are normally 35 days, which puts me at 5 weeks 4 days. my first HcG on 7/6 was 180, and my second yesterday was around 380. i just got a call from my OB that they want to move my ultrasound from 7/22 to 8/4, because based on my HCG i’m not as far along as i should be. based on my hcg, they would say i’m between 4-5 weeks, not 5-6 weeks. anyone else go through this?