Malakh is the Hebrew Bible's general word for a messenger. It is used of human envoys throughout the historical books, of a prophet in at least one passage, and of the beings later readers call angels, without any change in the word itself; the Greek angelos that gives English its word for angel is a translation of exactly this ordinary sense. Nothing in the vocabulary marks the difference between a king's courier and the figure who stops Balaam on the road. Context does all of the work.
That generality is the point of the word and the difficulty in it. A being defined entirely by an errand has no nature to describe, no fixed form and no independent standing; the malakh of a story is simply whoever was sent. Several of the most striking encounters in Genesis and Judges turn on the ambiguity, with the messenger addressed as the sender partway through the scene and the narrative declining to explain the shift.
When the ranked lists were compiled, this general term was narrowed into one order among ten. That is a real change of meaning rather than a tidying of terminology, and it is one of the clearest illustrations of how the hierarchies were built: existing scriptural vocabulary, not written to describe a hierarchy, sorted into one. A reader who meets malachim as the sixth rank and then meets the same word used for angels in general has not found an inconsistency in the sources, only two stages in one word's history.