You don't even necessarily have to predict earnings. If you've ever connected to your financial account via a service like Plaid or it's ilk, that service has an API endpoint[1] they can call to neatly package up your income information. Sometimes it seems innocuous and unassuming for a one time use like identity verification, or to set up automated payments, or a one off transfer/disbursement. Other times it's for stuff like getting a consolidated view of your personal finance (i.e. a transaction aggregator such as Mint). But if you authenticate for anything, that service has access to everything.

And unless you rotate your financial passwords on a frequent basis, that access continues pretty much indefinitely[2].

[1] https://plaid.com/products/income/

[2] Not true for 100% of cases, but a general rule of thumb that's applicable to the majority of institutions they log into with your credentials.