Why are real and nominal total effective tax rates different?

In the Tax Lab, the 'Effective Total Tax Rate' for a plan either in a given year or for all years of the plan is calculated as:

Effective Total Tax Rate = Total Tax / Total Gross Income

Total tax is taxes from all sources, including excess Medicare premiums due to IRMAA (but not base Medicare premiums that are not income-level related). Total Gross Income is all income that could in principle be taxed, whether it is taxed or not. For example, Roth distributions are included in Total Gross Income, as are all Social Security benefits (whether they are taxable or not), withdrawals from taxable investment accounts (including basis, which is not taxable but would be taxable if it were gains), and much more. The goal of this value is to show how much of a household's income (money used to fund life, as well as passive income like dividends that are taxable whether or not they are spent) goes to taxes. We don't use taxable income as the denominator here because taxable income is affected by deductions. If deductions go up, the effective total tax rate will go down - as it should.

On a year-by-year basis, the Effective Total Tax Rate is the same whether you are looking at real (today's dollar) or nominal (future dollar) values. However, for a full plan's effective total tax rate, that isn't necessarily true. If the amounts of tax and taxable income are inconsistent through a plan (as is often the case), that lumpiness will result in different real and nominal Effective Total Tax Rates, as in the example below.
 

However, if the tax rate is uneven or lumpy because of differences in the makeup of income in each year (Roth distributions vs IRA distributions, IRMAA vs no IRMAA, differences in taxability of Social Security), the Effective Total Tax Rate for the full plan will not be the same in real and nominal terms. Here's an example.
 

Here, the Effective Total Tax Rate is the same for real and nominal tables when comparing year by year. But the totals for the overall plan yield a different rate when viewed in real vs nominal dollars. Her,e that difference is small: just 10% vs 10.2%. But in a multi-decade plan with a lot of lumpiness (such as with Roth conversions), the difference in effective rate for the full plan between real (today's dollar) and nominal (future dollar) Effective Total Rates can be much larger. The 'real' version is probably a better representation of the tax load on the plan. However, the 'nominal' version does better reflect the feeling for the client of paying those actual taxes using the dollars in their wallet (or, more realistically, bank account).