There are probably two factors at play here:
!)
Sampling error. I played football in high school and college. My high school team tended toward average or slightly-below average. Yes, there were a few on the large side, but not that many or that big. Occasionally, the swimming team would have to shower with us. They were, on average, noticeably bigger and tended to be on average to above-average side. Fewer on the small side, more on the big side. It was noticeable; there were a few jokes made about the fact. The same was true of the guys on my college football team. They tended towards the larger side of things, including a few absolute porn-size monsters.
It's that we're dealing with a small population here. The guys on the college team across town might just as easily have been hung like my high school team. When you're dealing with thousands, evenmillions, the variations in size even out.
2)
Bias. Guess what? Many gays tend to be size conscious (if not size queens). Many smaller-hung guys may stay home. Or some may sense another's a size queen and steer clear of him. I used to go occasionally with two friends to gay bars. Even with my limited experience and interest), I could often pick up what many guys were interested in. There were often verbal and visual clues. Or some guys' blatant display of their large size may have intimidated them. I'm good-looking but hung like a chipmunk and was sensitive to these things.
The studies you cited which show cocks in the mid-to-lower five-inch range dealt with much larger sample sizes, and were screened to be representative of the larger population. You obviously didn't do the latter, although you may have attempted the former.
But how many of them have been as small as 5.1 inches??
I'm 1.1" smaller. And, according to your measuring pics, you're about 0.5" smaller yourself. We just weren't in your sample. As AMTZ410213 said, "You're just lucky."