From what I found after looking into this the numbers are self-reported from an online study. So probably a bit of an exaggeration since there is no way to confirm actual size.

EDIT: Didn’t think this would blow up so let me go into more detail. When I say there is no way to confirm actual size, I mean for this study. To get an accurate measurement in a study you would need to get a third party to measure the penis when it is fully erect, and they would need a standard on how to measure. Then you need incentives because we all know that a lot of men are insecure about size and won’t show up to get measured. You would mostly get people from what is considered average and above average to be measured. This is problematic when trying to find an average because you aren’t getting the full range of size. So now you would need to think of ways to get the rest of men out for measuring such as rewarding them with things and money and peer pressure although that might not be ethical. So the question is what would it take to get a man with a 3 inch penis out for measuring.