It's a matter of the best results that can be attained. The legal system can choose between being unambigious (which requires very complicated language) and being understandable to an ordinary person (which requires simple language). It is impossible to have both.

The problem is that any system which chooses the latter option is itself unjust as it will inevitably create ambigious scenarios. You then have to choose between generousness in interpretation whenever an ambigiuity exists, ignorance of the true meaning of the law as an excuse, and having to convict people ignorant of how to interpret the law as there is no correct intepretation to apply to the facts. The alternative of 'the spirit of the law' is illusionary, as shown by the massive amount of bias humans have interpreting such a vague concept.