It's true they use version numbers internally. It only makes sense for related product lines. Their use of version numbers isn't the same, though. Each release of Windows is a unique, standalone, new edition, with its own marketing teams and development teams (though there is obviously sharing) until prior versions are completely removed from the support lifecycle.
WoW isn't that. WoW is still one game. Just one. They've never made a second. They've expanded the game they have with a bunch of patches and expansions, but the core is still that first World of Warcraft. Until Blizzard says "this is our second version of WoW", there is no WoW 2.