Can We Just Trust Measurement Unit e.g. like size number 30?

We know we can't test the fitting of the trouser in Taipei's Night Market as there is no such thing called Fitting Room on the street.

So Uncle8888 bought along a pair of his trousers to benchmark the size number 30 to the RIGHT FIT!

Now, who says no need to do benchmarking?

Same as your investing. Right?

How do we know we can right fit that number when we don't benchmark?