This results simply from the nature of D&D being a) written in English and not in mathematical or logical notation and b) not written by lawyers or programmers. The English language (and every other non-constructed language) contains ambiguity by its nature, and people make mistakes.
Ah, the common Flawless Law fallacy.
If lawyers could write non-ambigous laws, then we wouldn't need courts and tribunals, and laywers wouldn't have to need to study years just to try to grasp their own rule system. And judgements wouldn't extend for years or even decades.
So, if laywers and politicians who get paid fortunes for their work still put out a crappy system for the behaviour of a single species whitout magic, what hope does Wotc designers do of perfectly avoiding ambiguity when they have to mix in multiple species and magic and dragons?