One is Japanese, the other is English. Terminology. Sort of like Chinese fun vs. English rice. And I think it is helpful to think of kombu as a sub-type of kelp. Rausu kombu is thick, nearly inedible and used exclusively for dashi stock, whereas rishiri kombu is the same plant thinner and less mature that is a common ingredient in stews and pickles.
Rehydrated, kombu throws a lot of people off because it is oceany and slimy. Also because it is a natural source of monosodium glutamate.
One of my first chores as a kid was to go forage at low tide to harvest aosa, or "blue grass." It's actually an algae, but the USFDA requires that it be labeled as "kelp."