>>57995495 (OP)>>57995649It's an old, obsolete classification.
It's the same reason Blastoice is the Shellfish Pokémon. In Japanese, they use an old Chinese classification that separated all aquatic animals with shells (clams, crabs, snails, turtles) intended for human consumption. This was poorly translated into English. A more correct term would be Water Shell Pokémon or simply Shell Pokémon.
This is also the case with Bug type, the scientific term only refers to certain hemipterans, informally in medicine it refers to certain pathogens, in computing to software errors, but the old jargon means all kinds of crawling creatures such as spiders, centipedes, land isopods, snails, worms, insects, even sometimes small lizards and frogs are included.