Hcg vs Dating scan

Nicole • 28yrs old & TTC #2
Hi ladies...
I need some advice or explanation or something lol. 
My hcg levels come back 7500 which ny dr said i would be 7 weeks.
My dating scan showed 5 weeks...
Isnt ultrasound more accurate? Could i just have higher levels of hcg?
Its been 10 years since i had my last child and i cant remember this stuff...