Why is "United States Chained Consumer Price Index" trending?

Wikipedia Overview

The United States Chained Consumer Price Index (C-CPI-U), also known as chain-weighted CPI or chain-linked CPI is a time series measure of price levels of consumer goods and services created by the Bureau of Labor Statistics as an alternative to the US Consumer Price Index. It is based on the idea that when prices of different goods change at different rates, consumers will adjust their purchasing patterns by purchasing more of products whose relative prices have declined and fewer of those whose relative price has increased. This reduces the cost of living reported, but has no change on the cost of living; it is simply a way of accounting for a microeconomic "substitution effect." The "fixed weight" CPI also takes such substitutions into account, but does so through a periodic adjustment of the "basket of goods" that it represents, rather than through a continuous adjustment in that basket. Application of the chained CPI to federal benefits has been controversially proposed to reduce the federal deficit.