Consider several proclamations issued by a ruler. One concerns an administrative matter, another addresses a particular audience, and a third communicates general instruction. When collected together, each proclamation retains the form suited to its occasion.
This is an analogy about literary arrangement, not an equation between divine revelation and human commands. Its specific point is that independent communications made on different occasions need not be rewritten as chapters of a systematic treatise.
The analogy illuminates variation in the length, opening, conclusion, and combination of subjects within surahs. Their differences of form do not negate the unity of revelation or the recognizable integrity of each surah.