The game is sold for 10 euro a copy.

600,000 / 10 = 60,000 copies sold in one month.

1 month = 30 days

60,000 / 30 = 2000 copies/day

That means, on average, the game sold 2000 copies every day of the last month.

Is that a reasonable number?

On the minecraft site, the number of copies sold in the last 24 hours is constantly shown. As of this post that number is 4486. So it is clear that 2000/day is well within the realm of possibility.

This isn't even a particularly massive number for a successful indie game. World of Goo was on sale last year (a year after it was originally released, I might add), and made 57,000 sales in a single week. (Due to the nature of that sale, about 17,000 basically got the game for free, so a more accurate number is around 40,000 sales over that period.)

So no- you really don't need a conspiracy theory to explain these numbers.