Why, at 175 some people consider you as short and others as tall

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Do you think that people are underestimating what is 5'9" ?

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In my case there is a friend who told me that I'm lying (I told him that I am 5'9 - but then he went near me and said ok, but still if I can't say that I'm 5'9 ... he was probably thinking I am the same than him even if I have my chin at his eyes level and he told someone same as me he is tall - but that guy is always denigrate), and also my grandfather told me I am about the same that my 6'1 uncle. It's weird, I still don't know how to consider myself in others people point of view.
But also what I know is when you say your height, it's changing people point of view a lot. That's why I'm avoiding that because like that some people are imagining me at 5'10 / 5'11.

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We already talked about that famous average in Netherlands, that's bullcrap. Honestly, people in Germany or Czech Republic are not shorter than Dutch people. That's a myth. 
Also that big average (6ft) is ONLY FOR MEN, average for women is 5'6 so stop kidding
the biggest woman average is Estonia, go there

In my country maybe a bit, but a gap of 7 cm in average ? it's too big