how far along in i?

I am so confused with trying to figure out how far along I am. this app tells me I'm 6 weeks and 5 days another app tells me I'm 7 weeks and then one more app tells me I'm exactly 6. I don't have my first doctor's appointment until August 6th so I know I'll for sure no by then but I'm just so confused on how they get so many different dates if they just go by my last period.