It's not so much an error, to be frank, and more of a deliberate manipulation.
Everybody and their dog for quite some time "knew" that inflation is bad, because you get less for the same money when prices rise.
But, around the world, it's usually some government entity publishing inflation numbers... So it isn't a big surprise that the governments came up with ways to keep the inflation numbers as low as possible, at least on paper if they failed to keep the number low in the real world. One of the many ways to artificially "shrink" inflation numbers is Hedonic Quality Adjustment.
A government does that to avoid disgruntled citizens asking questions about why the inflation is so high, but you also does that to avoid disgruntled companies asking questions about why the inflation is so high and thus why their workforce keeps demanding wage raises to offset that high inflation. And then there are those rating agencies judging the risk of state bonds and consols also in part on the inflation numbers.
Another potential way to massage inflation numbers is the "inflation basket"[1]: E.g. the UK between the 2017 and 2018 basket the gov't slightly decreased the weight of all goods in that basket (such as "Food" and "Health") except for "Alcohol & tobacco" (steady at 3.3% weight) and "Housing & household services" (27.6% to 30.0% weight). Now I'm perfectly sure there is a real fine explanation why this happened other than to optimize for low inflation numbers on paper, just like I'm perfectly sure there is a real fine explanation why Hedonic Quality Adjustment just makes sense and it's absolutely necessary to use it in your calculations!!
Other countries do the same or similar things.
Governments got really good at mucking with numbers. E.g. in Germany they started massaging unemployment numbers by essentially redefining what "unemployment" means[2]: It used to refer to people seeking jobs but now it's those job seekers minus all those people who are enrolled in government "employment" programs (where e.g. they teach you how to write a good CV), working in shit jobs that don't pay a living wage (incl "1€ Jobs") and therefore still rely on the welfare state, etc. The most drastic "adjustment" they came up with in regards to unemployment numbers in Germany is that they simply do not count anymore unemployed job seekers older than 58 years as "unemployed" (retirement age by law is between 65 and 67 years old, depending on when you were born).
Other countries do the same or similar things.
[1] https://www.ons.gov.uk/economy/inflationandpriceindices/arti... and https://www.ons.gov.uk/economy/inflationandpriceindices/arti...
[2] https://www.tagesschau.de/wirtschaft/hg-arbeitslosenzahlen-1...