This is obviously more of an opinion page, but I felt the need to write this somewhere.
You might believe that this website is wrong and that there's no way the average size is around 5.5", as it is clearly 7"-8". Right? Well...
Even if everyone around you says they're [insert number here] inches big, it's probable that they're either exaggerating a bit too much or outright lying. They might not even be trying to exaggerate, but they can end up doing so by not measuring themselves properly, rounding up too much or being fooled by other's perceptions (the commonly referred "girl inches" vs "real inches"). Secondly, unless someone took out a ruler or other measurement device and measured their partner, it's likely for them to believe whatever size their partner says they are, further spreading the myth due to a lack of verifiability.
Same thing goes for porn: using camera tricks, smaller actors and actresses, they can make a smaller size seem bigger than it actually is. Many things common in that industry are flat out wrong in real life. If not enough information is provided about an environment, the brain can make some somewhat crazy assumptions. This video from Vsauce on YouTube talks about a similar yet unrelated effect, the "Moon Terminator Illusion" where people perceive distance wrongly without the right context in front of them. Skip to 2:17 if you want to go straight to the point (and watch until 4:19). Also, have a look at this (safe for work) image gallery with three paper rolls in sizes 7"x5", 7.5"x5.5" and 8"x6" (already huge!) compared to nearby objects. It's very easy to make them appear larger just by using clever angles.
Perspective can truly be deceiving. You normally view yourself from a top-down point of view, with other elongated prats of your body nearby (due to foreshortening) as the only way to compare. Anyone else is going to see it across from them, not foreshortened and with enough "clues" about the environment. You can try comparing yourself to other objects to have a better understanding of its real size, or make a paper roll roughtly its size and compare it that way.
This page made by someone else has more details on the matter: Unraveling Size (NSFW)
Other common beliefs are that microsized members lower the average significantly, and that these datasets compare people from ethnicities where the average is smaller.
Of course, you'd need to read each and every study here to confirm this for each individual one, but for the most part, they exclude from the database people who have complained about having a microsized dick. Their re-inclusion could only drive the average in each study down, not up. About comparing against other ethnicities, generally all of them fall in mostly the same range, but if you're still unsure, you can see for yourself the differences in the Dataset List page. It documents the average for the studies used in calcSD and other information about them, such as the country they gathered data from. In the "Veale et al." study, the countries are listed from the ones that had the highest amount of people contribute to the dataset to the lowest.
There are other claims such as "there's more people on the low side of the average in these studies than in real-life" and "most people on the high side of the average are more sexually active". There's currently no evidence proving or disproving it, so...you can believe in it or not, but personally, I don't think those are the case either.