Core CPI isn’t headline CPI. Its an additional measure that is published alongside the headline CPI; the removal of components with a high non-seasonal volatility [0] is thought to potentially make it more useful in providing early indication of changes in longer term trends; literally no one presents it as a better measure of current inflation.
[0] the components involved also have a high seasonality, but that can be addressed via seasonal adjustment, but the high non-seasonal volatility means that fluctuations in those components can drown out early trend indications in ways that seasonality adjustments can’t compensate for.
[0] the components involved also have a high seasonality, but that can be addressed via seasonal adjustment, but the high non-seasonal volatility means that fluctuations in those components can drown out early trend indications in ways that seasonality adjustments can’t compensate for.