Factorio Forums

The article in question is https://wiki.factorio.com/Tutorial:Qual ... cling_math

The text above the table says

The columns "X products" denote modules need to install into the machines that are set to produce items of X quality.

However, looking at the table it recommends using quality modules to produce normal quality products, which makes no sense. It is also worth noting that none of the legendary quality products have quality modules recommended. So, either normal and legendary quality are swapped, or the table means to say "craft legendary items starting from X quality product/ingredients", which means the highlighted text is wrong.

There are no mistakes in wiki, but looks like you have weird interpretation.
"set to produce items of X quality" means normal quality recipe is chosen in machine. Recipe quality is not about target quality but about component quality. However you erroneously interpret it as "goal is receiving normal quality items". Goal for all calculations in article is opposite - get as many quality items as possible. And it is quite logical that when you produce from already legendary components there is no sense to put in quality modules, they will do nothing. That's why production modules are used.