I honestly have never measured myself. It's like the question you ask your SO about how many men she been with. When she gives you the number multiply it by 3 and that is the reality. When you ask a guy how big he is decrease it, because every man is over 9 inches on the internet...lol.
Sidebar, I read that there is only a certain length that a woman has before you are reaching the cervix (which is 3 to 5 inches depending on the woman). Therefore, anything longer is just wasted. Now on the other hand, girth is different. Depending if the woman likes to be stretched, could be an issue.
Just my 2 cents, but like others have said it is how you work it. The ones that measure might be trying to compensate for other things they may be lacking elsewhere. Like I said..Just my opinion in my little piece of my world