So I got my blood work back !

Soooo they called me back and I'm estimated at one to two weeks early pregnancy !! Happy for the confirmation, but on an online preg calculator based on the first day of your last period it estimated me at 3 weeks 3 days. How is the blood test less ? Maybe the hcg levels are very low ? I'm supposed to get more blood work next week . Trying not to get nervous and stay positive !