This isn't tax, you'll still have to pay income (or whatever) tax on the $5,800,000. This is them deciding to give you less money, and claiming with careful investment you could make $10,000,000 over the next 40 years.

I think it will always make sense to show gross salary. Various things can affect tax rate, such as 401k and HSA pre-tax withholdings, and the income of your spouse.

quite frankly, yes, but that is an entirely different conversation. :)