Probably because the online version was more expensive than the physical copy (on Amazon). If comparing like for like the digital version was way overpriced.

This graphic single handedly made me become a pc gamer.

First point is they count the WoW year passes as sales, even though they were free to those people.

Either way, the game is just a hype fest. Doesnt really deserve all those sales, but blizzard drones cant help themselves.

That and victims of the hype and advertisement machine.

If you look at it objectively and watch a game like Dungeon Siege 3 didnt have sales anywhere near, even though it looks and plays the same (maybe even better) does tell you that something else was at work here.

They are great at convincing people that a turd is made out of gold, i'll give them that.

3 Platform to sell that much MEH

DS3 was a worthless PoS that was basically unplayable on PC and had the worst loot of any loot game I have ever played. Not to mention multiplayer is totally broken, I mean who's bright idea was it to make you have to be on the same screen in multiplayer? Fk that game, it plays worse than DS2 FFS

That is not even mentioning the crappy level design and quests 

Dungeon Siege 3 was a horrible port that was essentially broken on PC due to terrible controls. It also took the previous Dungeon Siege games and completely ignored them. if anything, it was an average hack n' slash with bland level design and was unforgivably short. Couple that with the fact you can't keep your loot for further campaigns/gameplay and you can see why it didn't sell. I haven't even mentioned the broken multiplayer system.

Diablo 3 at least nails the fundamentals of the genre. Does it need more work? Of course it does (nowhere near as much as people make out), the end game isn't satisfying enough for instance, but Diablo 2 also needed patches and updates when it was originally released. People know that games like Diablo, Starcraft and WoW will be supported and improved upon for years to come.