There could be some bias. Participation in such surveys is usually voluntary. It could be those who do well and who get good jobs responded to the survey. Those still unemployed or in low paying temporary jobs might have chosen not to respond; they are under-represented.

Thus the results are skewed towards the more optimistic side. Especially if the survey is done by recruitment agencies, they are incentivised to dangle a carrot that is larger than reality to attract people to apply for jobs. Tertiary institutions also tend to play up the employability figures and salaries to show their graduates are highly sought after in the job market, to attract potential students. Even figures provided by government may be massaged or presented in some way to manage public sentiments or for political reasons.