Think about it --- they're trying to sell you something.Most manufactures sell stock sets as "furniture" particularly if its a multi-piece stock. While that may have been the case originally the term has evolved, like many other older firearm terms.
Yeah, the term furniture evolved just like "casing" evolved from case. Casings are what sausages are stuffed into while cases are the backbone of a cartridge and what seals the chamber. But you still you'll see "casing" misused all over the place too. Bottom line is "furniture" is from a bunch of yanks misusing a Brit term in an attempt to sound smarter when "stock" is the better, more precise, and easier term to use.