This thread is fascinating. I've never given so much thought to the size of the average penis, erect or otherwise, in my life (or at least since the 8th grade). Just fyi: I've seen plenty of porn videos, and I don't remember the actors being bigger or smaller than any other... I think it's all a conspiracy to sell penile enhancement pills!
A lot of guys measure their cocks along the bottom of the shaft, which will give them extra inches depending on where they stop. The correct way to measure a cock is from tip to pubic bone, along the top of the shaft.
I think some of those guys took pictures of "scared turtles". If the temperature is cold or hot enough, they all look like that. A cold day can make any man look like an 8 year old boy