Doctor says I’m further along than LMP?

I had HCG blood work done at what I thought was 4wks6d (my estimate based on LMP and my slightly shorter than normal cycles) and she said I was likely 4 weeks based on LMP. They referred me to an OBGYN (first pregnancy, didn’t have one so I went to PCP) and they called and set up an ultrasound for June 27th and first prenatal appointment on July 25th.

The nurse called me a bit ago to let me know the results of my bloodwork. Apparently, according to those results, I’m 6 - 6 1/2 weeks. I knew my HCG had to be quite high based on how early I got a positive (3 weeks 2 days) and how strong my positives are. But, I’m confused as to how I could’ve gotten pregnant before my lmp? New to the whole pregnancy thing & not sure how to take this. The nurse said she wasn’t sure but she would ask the doctor about it and call me back.