Why the same dollar is the only fair comparison
A $7,500 traditional contribution costs you $7,500 of pre-tax pay. A Roth contribution of $7,500 costs you $7,500 plus the tax on it, so it really costs about $9,600 of pre-tax pay at a 22% rate. Comparing the two balances directly would give the Roth an unfair advantage.
This calculator equalises the pre-tax cost: the Roth and taxable accounts receive what is left after tax is paid, so all three columns start from the same pay packet and can be compared line by line.