this is the survival bias: those who earn little has no or negative incentive to post while those who earn more has strong incentive, and they may, along the way, inflate the number a little bit, like maybe report their salary with the expected increment as their current salary.

bottomline: the income reported here is skewed on the high side. the best guages are the MoM and IRAS numbers. MoM do have numbers by occupation, and IRAS by age. You cannot deceive the tax man.