How do living costs differ between households?

Why can these differ from CPI?

These living-cost measures include mortgage interest. CPI treats owner-occupied housing differently, including new dwelling construction costs rather than mortgage interest. Neither measure includes the purchase price of land.

The living-cost indexes use an outlays approach: they focus on expenses incurred to maintain a household’s living standard. CPI uses an acquisitions approach: it records prices of goods and services acquired in the period. The different treatment of interest can make the measures move differently.