Healthcare spending in retirement is rarely modelled properly, and it lands unevenly across the decades.
Most retirement projections treat healthcare as a flat annual line. In practice the cost curve steepens sharply in later decades.
Premiums, surcharges tied to income, and long-term care each behave differently, and only the first is easy to forecast.
Building a separate reserve for this category, rather than absorbing it into the general withdrawal rate, tends to make the rest of the plan hold up better.