People need to understand something important about Publishers and PC sales: They don't only mean Direct Steam/Epic/GOG, they also mean their advance sales to the PC key sites (legit and grey).
Yesterday I looked up Indiana Jones on GMG to see if they have an XP offer, but the game was "Out of Stock".. That would imply that they keep a stock of keys for every game, which is sold in advance and topped up when stock runs low or runs out entirely.
It's basically the old-school publisher/retail agreement, but played out in digital form.
This means that Microsoft could have sold 10,000 keys to say, GMG or Gamesplanet alone, and pocketed a portion (or all) of the money, meanwhile the game "only has 12k CCUs". Now apply this same logic to all games that are being sold on PC.
Steam CCUs only tell the full picture when the game is F2P on Steam, because then you won't be "buying" the game from a non-Steam store, and even then it only tells the Steam portion of the story.