Another reason the figures might be skewed, this survey, like a lot of others, asserts that stretched cock length is equivalent to erect length, i'm not sure that's true. Sure, some scientist might have checked with a number of guys comparing the stretch with their erections and found a correlation good enough to 'proove' it but I doubt all those who participated were fully turned on.
My basic erection is way smaller than it is after 10+ minutes in a woman or even wanking. When I first get hard I can touch my index finger and thumb around my cock, when i'm really hard I can barely touch my middle finger and thumb, that must be a half inch difference. I'm pretty sure I get longer too. I can't be alone on this.
I just don't think the guy that proved stretched length = erect length was seeing all the guys at their best.
Also, if they were using stretched length to avoid erection quality messing with the outcome, how did they even get girth measurements?