Well seeing as how Breath of the Wild is being released on 2 systems and being a launch title for one of them, and history telling us that the last Zelda game to do that - Twilight Princess, has sold close to 9 million copes overall, over 7 million off the Wii version, alone, AND if Switch sales are being included along with Wii U sales, then I think it's very easy to see why people would think Breath of the Wild should catch up to Halo 5.
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