If you have not been receiving EIC, more income can still lower your refund depending on your withholding. A refund is the balance left over after you compare the amount withheld for taxes with the amount you actually owe. Sometimes when you get a raise or change jobs, your withholding might not keep up with your wages. This is especially true if you have two jobs, since the withholding system is not very smart and will under-withhold if you have two jobs, unless you make special adjustments to your withholding.
