The MediaWiki software is developed collaboratively by people from all around the world. To help facilitate this process, standards of desirable practices have been established.
The "policies" describe what developers must do, and the development "guidelines" typically describe what developers generally should do. There can be no deviations from policies; but minor deviations from guidelines, like coding conventions, can often be allowed based on individual circumstances. Whether there is good reason for a given deviation that is worth the downsides of conformity to the established norms is a matter for judgment of developers and their code reviewers.