This is because the IRS has a set of ordering rules that govern how the funds in your Roth IRA must be withdrawn.

FORBES: Taxes From A To Z (2013): O Is For Ordering Rules

The last bit of funds to be withdrawn under the ordering rules are the returns on investments or earnings on contributions.

FORBES: Taxes From A To Z (2013): O Is For Ordering Rules

When a state tightens Medicaid eligibility rules, when a doctor refrains from ordering an expensive test, when Congress covers the drug bills of a 65-year-old but not a 64-year-old, that is rationing.

FORBES: A Price on Your Head