I have worked places with salaries that were public knowledge and non-negotiable, and places where salaries were hidden and varied wildly. My mentor recommends

I have worked places with salaries that were public knowledge and non-negotiable, and places where salaries were hidden and varied wildly. My mentor recommends finding out the compensation of those you work with, and use that when you negotiate for a raise. It also sounds like the HR person is the one with the real knowledge here, and they are the one in the best position to report this if they are willing.