CPI is inherently flawed due to basket of goods methodology and consumer substitution for cheaper/different alternatives than in the past when prices to their current basket changes. For example average housing spend could be steady while people get smaller homes and CPI would not reflect that. Likewise with lower quality food by some metric or other.

So much like how the S&P/NASDAQ has a bias for growth because losers are swapped out for winners, the CPI basket has a negative price bias as expensive goods are swapped out for cheaper ones.