They’re tracking the value of money. RPI/CPI aren’t only tracking the prices paid, but also the quality of goods exchanged. For example, TVs used to be bulky b

They’re tracking the value of money. RPI/CPI aren’t only tracking the prices paid, but also the quality of goods exchanged. For example, TVs used to be bulky boxes, with a small, low res, black and white screen. These days they are flat and high res with a large colour screen. They’re better TVs. So part of the price difference between TVs now and TVs in the 1950s is that money is worth less, but some of it is that TVs are better. If you’re interested in tracking how much your currency is worth less these days, you need to compensate for this. On the other hand, if you want to buy a bottom of the range TV… you can’t buy a small bulky black and white TV these days. You’re going to buy a more expensive colour flat screen, and ok it’s better, but it’s also costing you more. (This also applies to fridges, freezers, washing machines etc.) Tldr RPI/CPI track changes in the value of money, and understate changes in the cost of living due to quality improvements in the goods people buy.