I'm 5'11 but taller than people who claim to be 6'1. I read some statistic out there that says that dudes tend to add two inches to their height. Makes sense because you can probably get away with adding two inches but going with three or over will probably get you the 
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That's not the only thing they add 2 inches to.
In both cases there is rarely anyone who can bust out a tape measure and find out if they're bullshitting.
I can understand guys my height adding an inch or two. 5'11 is considered short but 6 feet isn't. However nobody can visually tell so you might as well just add that imaginary inch.
Regarding your other point, it reminds me of a story in high school. There was a black guy on our football team who was insecure about his penis. Nobody gave a shit but he had to make sure it looked like he had a big dick when we all showered after practice. So he would play with himself a little before going to shower. For growers, doing this can add a few inches but still remain flaccid. Well, someone caught him doing that and he never lived it down until he graduated high school.
