Confused on how far along?

Okay so I had my first Dr app appointment today and I'm sorta confused. I was on my period from January 30th until February 3rd and i got pregnant somewhere between the 5th and the 10th. That would put me at right around 7 weeks give or take a couple days. My doctor said that judging by my period I'm at 8 weeks tho.. And on one of my other pregnancy apps I put in my conception date as the 10th a d it says I'm like 8 weeks 5 days.. Probably a silly question but I don't understand the math there, how do they determin how far along you are??