Over time, languages, social settings, and patterns of usage change. As diverse communities entered Islam, some older Arabic idioms became less familiar and customary meanings shifted. Expressions that were readily accessible to the earliest audience could thus require more study by later readers.
The difficulty need not be a defect in the discourse. It may arise from distance from the original linguistic environment, lack of historical information, or unfamiliarity with a rhetorical mode.
Even a correct basic gloss does not automatically reveal whether a word is literal, idiomatic, or allusive in a given verse. Context, grammar, and sound transmitted interpretation must work together.