There's plenty of uncertainty when comparing numbers when there's uncertainty in how the numbers were generated. IQ tests aren't completely uniform across space and time. Worst case, 170 IQ Joe might have taken some random online test.

I think the simplest explanation is the Bayesian one: most people who introduce themselves using their IQ are socially inept, so introducing yourself using your IQ is Bayesian evidence of social ineptitude and other unpleasant things.