The information available via the public record is not as detailed (typically annual salary)and not definitively tied to any person. The Work Number is tied to your SSN and is much more detailed than the public record (each paycheck and a breakdown of different compensation).
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.