Just to be absolutely clear: the “top” is the side closest to the belly? I wonder how many men actually measure that way.
I would just like to point out, that the side being measured is not the side women usually see. Or indeed, not the side anyone but the owner of the cock is likely to see. When you look at it from the outside perspective, the base of the erect cock is going to be indistinct. I would think a penis might look one to two inches longer than it measures when measured as @uberbatman suggests.
In any case, when estimating it’s size from the outside, I think that people will think a penis is larger than it measures using the prescribed technique. I don’t know what this means, but I do think it is interesting.
In addition, everyone is on the honor system here. You know. She offered her honor; he honored her offer, and all night long he was on her and off her. I think that might have an effect on the validity of the data and the accuracy of the measurements. I can’t say whether I think this would tend to bias the numbers up or down. But maybe someone else has an opinion. ;-)