The tool calculates taxes.
From Fidelity's "Detailed Methodology" PDF:
"...The Total Expenses amount includes estimated taxes...
...The Tool estimates federal, state, and local individual income taxes
on, among other things, investment earnings, distributions from
tax-free and tax-deferred retirement plans, Social Security, and any
earned income or salary...
...tax calculations for any particular year are estimates based on information provided by you, tax rate data supplied by third parties, and projected inflation adjustments to tax brackets, among other things...
...The Tool makes certain tax assumptions based on the types of
income, accounts, or other information entered. For each year of
the projection of your financial plan, the Tool estimates your effective tax rate based on projected taxable income..."
and yes,
"Health Care Costs: The default inflation rate of health care costs
is a schedule of rates, which start at 4.9% for some time and
slowly decrease to general inflation, based on Fidelity research..."
Given the above, it appears that changes in total expenses will vary based on assumed taxes and changers in assumed health care inflation, which will change based on what accounts money will be coming from as well as what other tax assumptions made.