Does it matter whether the income is salary, or additional voluntary whatever, or bonuses?
So that famous sketch has become (it would appear in some people's minds, not all):
"I look down on him. His skills are not in as short supply and so he does not deserve to get paid as much as me
I am entitled to get paid lots, it is only reasonable to want to earn as much as possible
I look down on him too, he earns a lot more than me. So he is greedy and does not deserve it."