I Found Out I Was Pregnant Last Tuesday, Began Bleeding

I found out I was pregnant last Tuesday, began bleeding on Thursday and went to the ER on Friday, my HCG levels showed to be at 540 and estimated me at 4 weeks 6 days. I went yesterday to a different clinic and had blood drawn and my hcg was at 502... I began googling and started reading that different labs give different results. the ranges are completely different as well. ie. it starts at 3 weeks and the range is from 6-71 on the results from the 542... the second one 3 weeks range starts at 100 -5,000