Depending on the type of kelp, some may contain more of some minerals than others. For example, someone who has weak adrenals may benefit from the extra sodium in some types of kelp and people like me who already have enough sodium need more potassium in other types of kelp to conteract the sodium so I don't run so hot all the time.
Have you ever paid attention to how you feel when you eat foods high in either sodium or potassium. It could be a big clue.