Customer price index is based on average consumer behavior. The tricky thing is that consumer behavior changes when prices rise. For example, when the price of meat rises, consumers start substituting it with cheaper alternatives. If the old basket was $50 and the new one is $50, then there's no inflation.

Saifedean Ammous writes about this in his new book, the Fiat Standard. The chapter about fiat food is available online. [0]

[0] https://saifedean.com/fiatfood/