Iʾtilaf al-lafz maʿa al-maʿna means that the words chosen fit the meaning and occasion. Strong, weighty diction is suited to grandeur, force, or conflict; gentle diction may better express tenderness, affection, or lyric emotion.
This is not simply a choice between difficult and easy vocabulary. Sound, syntax, habitual associations, and semantic tone should work together. Excessively harsh diction can disturb a gentle subject, just as misplaced softness can weaken an otherwise forceful expression.