Blood test

I wasn't really sure where to put this, had blood work Thurs to check my hcg levels & have to do a repeat this Thursday b/c we had bd'd just a couple of days before that. My levels were between 0 & 6 & what I've read on blood work says at 3 weeks levels are between 5-50. Well what if I'm only 1-2 weeks? Wouldn't my levels be flat 0 if I wasn't pregnant? What's up with the 0-6?