What you are experiencing is selection bias.
When others brag about height, only the "tall" ones will be mentioned, because that's considered tall, and it adds to their social prestige. Do you want to be known as the person with only short friends?
As such, they will never mention their cousin who was 168cm, because it's embarassingly short and they will be associated with short genes.
This results in a sample that is overwhelmingly biased towards tall references. While averages are rising, there is very little chance that youths in Shanghai or Beijing average more than 177/178cm.