The SDs there are taken from the Lifestyle's study, but using the median instead of mean. When people are interested in the 'average', they generally mean to ask what's most common, not what the literal 'sum of all numbers it set divided by total numbers in set'. Being strict about averages can get downright nonsensical, e.g. saying that if the average family has 2.5 kids, then you're an average family if your third kid is born without half of their body.
I do think there probably are a lot more 7.5s than that distribution would suggest, which is what I meant when I said this model breaks down quickly at the tail ends. If you look at page 5 here:
http://www.scielo.br/pdf/jped/v83n5/en_v83n5a08.pdf
you'll see in the graph that there's about a full 1" between the 50th percentile and the 90th, but only about 0.5" between the 50th and the 10th, ie. the vast majority of 'medically normal' dicks smaller than average aren't really that much smaller.
My numbers are really more trying to tie these data to perception. Going by Brazil 5.3" is about 25th percentile and 5.6" is 50th, but I think a 5.3" wouldn't really look 'smaller than average'. A full SD, so ~4.9"-5" BEPL would look noticeably smaller, and 6.3" would look a bit bigger than most. 7" would look a lot bigger.
