If the number of defending infantry units in a strong point exceeds the capacity of that strong point then the controlling player must clearly indicate which units can benefit from the strong point"
..suggesting that there are times when this difference matters.
Yes, this matters because we are talking about MORALE CHECKS. Each unit makes his own morale check, so it matters if each one of them are inside or outside the Strong Point.
However, later it says:
"The modifier for the strong point would only apply if all
defending infantry had the benefit of the strong point. The same rule
applies in infantry vs infantry fire, all defending infantry would have to be within the strong point to receive the defensive modifier"
In this case is doesn't matter because we are talking about FIRE MODIFIERS (Artillery or Infantry firing at Infantry that are defending the Strong Point area). These modifiers only apply if ALL units are inside the Strong Point (they are all under cover - if some are outside, then it makes sense not to penalize the attacker)