«كل إنسان ضاحك بالفعل لا دائما»
Every human is actually laughing, but not permanently.
The point is that, even when laughter is actual, it is not a condition lasting throughout human existence. A person may laugh at one time and not at another.
The negative construction analogously denies laughter under a given circumstance without making that denial permanent over the entire lifetime.
Actuality refers to occurrence, whereas non-permanence denies continuous persistence. One occurrence does not establish a permanent feature.