My question isn't about your surveys themselves, but rather if you have observed any bias in other surveys compared to your results.
A pessimist might believe that most compensation surveys are used by peer HR departments to set compensation bands and, naturally, they have incentive to systematically report those surveys low. Then they can point to these as "Market Rate" to cover their asses if challenged.
So is that intentional under-reporting happening, or do independent surveys mostly agree?