That's because most people who talk about it make it up or estimate anyway (outside of the penis oriented LPSG community). It's not "rounding" so much as they really don't know, or don't want to tell the truth.
I mean, looking at the lifestyles graph (
Image:Penis percentile.jpg - Wikipedia, the free encyclopedia ) a huge number (70% from a rough eye guess based on that graph) of guys are under 6 inches. Given 6 inches is the accepted standard average, do 70% of guys go through life thinking they are less than average? No, I imagine they just say they are average and never both to measure.
Yet most guys claim at least 6, if not 8, 9 or 12 inches!