Hcg levels

I don’t seem to understand these hcg levels. I did a blood test the first time and it showed that my hcg levels were on 6105 and I am 5 weeks pregnant. I did a test again a week after and it shows 1501 hcg levels and my nurse says I am 6 weeks 2 days pregnant. Exactly what could it be because isn’t the number supposed to be higher at 6 weeks