Aside from the girls in here that do measure...
Girls do have 5 inch cock men telling them they are 7. I remember when growing up all my mates were like "Yeah I'm 8" "Yeah I'm 9". I know for a fact that it turned out they weren't. One almost had a micropenis it turned out.
So girls get a bad point of view because of the lying men. I've had suprised looks from girls when I have told them I was only in the 7 ball park. They assumed I was bigger. Current girl guessed at 10.
The other point though is that girls often see the cock more from the underside. And If I measure my underside complete with the shaft that goes down into my ball sack, I come out at 10.
So maybe they aren't that bad at estimating!