Two retirees with identical savings and identical investment returns can end up with materially different spendable income. The difference comes from which account each withdrawal is taken from.
Accounts are taxed under different rules
Retirement savings typically sit in three buckets: ordinary taxable accounts, tax-deferred accounts funded before tax, and accounts where qualifying withdrawals are free of tax.
Each is taxed at a different point in its life, and withdrawals from each are treated differently in the year they are taken.
Because the buckets are taxed differently, the order in which they are drawn changes the total tax paid across retirement even when the amounts spent are identical.
Income in one year sets the rate for that year
Tax systems apply higher rates to higher bands of income within a single year, so concentrating withdrawals produces a larger bill than spreading equivalent amounts.
A retiree who exhausts taxable accounts first and then draws heavily from tax-deferred savings can face low-income years followed by high-income ones.
Blending withdrawals across account types can keep annual income within lower bands throughout, which reduces the lifetime total without reducing what is spent.
Deferred balances continue to grow
Money left in a tax-deferred account keeps compounding on the full pre-tax amount, which is an advantage while it lasts.
It also means the eventual taxable balance is larger, and in many systems mandatory withdrawals begin at a set age regardless of whether the money is needed.
Drawing modest amounts earlier, at lower rates, can prevent a larger forced withdrawal later at a higher one. The trade-off depends on the rates in each period.
Other thresholds move with taxable income
Reported income affects more than the tax bill. Eligibility for certain reliefs, the taxation of state benefits and the cost of some programmes can all step up at thresholds.
A withdrawal that crosses one of these boundaries can cost more than the marginal tax rate alone implies, because a separate charge changes at the same point.
Planning around the thresholds rather than the headline rate is often where the larger savings sit, and the thresholds differ substantially by jurisdiction.
Why general rules only go so far
Conventional sequences, such as taxable first and tax-free last, are reasonable defaults but assume a particular pattern of income and a particular rate structure.
Changes in circumstances, in legislation and in investment values all alter what is optimal, and rules set decades in advance rarely survive contact with any of them.
The mechanism is worth understanding in general terms, but the specifics vary by jurisdiction and change over time, which is where a qualified adviser earns their fee.