Three different gestation dates???

( on day of sonogram August 27th)
According to nurse I'm 8w4days
Sonogram pic says 7w6days
Lmp puts me at 8w1day 
I know lmp is probably the least reliable ok so that gets eliminated. But why did nurse give me two different days? She told me 8w4days while doing scan and when I got home and looked at pic I saw it actually says at the very top a different gest period "7w6days"