On credit card applications there are usually 2 questions regarding income. The first one in regards to the applicant's employment, and the second "total household income".
That "total household income" can be inflated to include the income of other person(s) living with you.
No credit card application asks for the source of the difference between an applicant's salary information and the information in the "total household income" segment of the application.
Can credit card outfits force you to reveal the names of anyone else in the household if you are applying in your name only and don't want to make anyone an authorized user?