Every company has people that say, 'Why am I getting paid less than Bob?' The reason is because the companies... actually value Bob's contribution more. We all think our own performance is better than it really is.
People's pay is a sensitive topic, and most people probably don't want to be told that they're already being paid what they deserve.
The difference between what someone is paid and what they think they should be paid is based on differences in perception of value ... and performance.