CPI Common Part Indicator. CPI Consumer Price Index. The number is actually available a couple of days before it is announced, and the Fed chairman makes sure to find out before the next meeting of the board to decide whether to tighten the tourniquet on the economy. That's how the price of goods is established for the purposes of CPI computation. Examples can be multiplied.
People buy more of things that are on sale at a reduced price.
They do this to save money. You would imagine that a hoity-toity macroeconomic indicator would be designed to take account of this. Every product that priced itself into a shriveled corner of the market in the last decade, they loyally continue to count. And it's the same basket all the time. Fruit out of season, buy yellow corn in February.