Dear Joy,
Since the product already put inside a validated cold box which able to maintain temperature 2-8C inside when the temperature outside the cold box not more than 30C. So the reason that why we mention 15-25C because we need to ensure that the cold box will not be expose the temperature above 30C. However in case that the cold box is put in condition 2-8C, it will cause the temperature inside the box go to below 0C which unacceptable for product storage. Hope this can answer your question.