One ranking says Hong Kong. Another says Singapore. A third puts Zurich at the top. That does not automatically mean somebody calculated badly. Cost-of-living rankings can measure different people, different spending baskets and different currencies.
An expatriate basket is not a local household budget
Mercer’s cost-of-living work is designed around international employees. That can place weight on housing, imported goods, transport and services commonly used by expatriates. A local resident may shop, commute and rent very differently.
Change the rent assumption and the ranking can move
Two people in the same city can face radically different costs depending on neighborhood, apartment size, whether they share, and whether housing is subsidized by an employer. Rankings that include premium expatriate housing will not describe every resident.
A city can become “more expensive” without local prices changing much
International comparisons convert local prices into a common currency. If one currency strengthens sharply, the city can rise in the ranking for foreign earners even when the café, metro and supermarket prices barely moved in local terms.
No one buys the average lifestyle
Food, transport, education, entertainment, healthcare and household services are weighted differently depending on the study. A family with two children and a car experiences the same city differently from a single remote worker using public transport.
Read the methodology before the city name
A cost-of-living ranking is useful when its household, basket and currency assumptions resemble yours. Without that context, “most expensive” is a headline—not a personal budget.