I think this is a little off, in that the data isn't coming from the various government entities or at least isn't required to be provided. I know at least in California most of the info is gathered by third parties using FOIA requests. It's also not associated with a SSN and just typically gives the annual compensation with limited categories. The Work Number on the other hand gets paycheck level details. Considering the data would be useless without a unique identifier, SSN is sent with it. Using The Work Number data, you could see pay period granularity changes to their compensation.

That is exactly what they are saying. The public records typically contain annual compensation, not the detailed breakdown available via Work Number.