That is also 1.7% of people who have played destiny.
Not 1.7% of people who have forsaken.
The game was free for a while too, there will have been loads of people who booted it up, played for 2 hours and then never came back. Similarly, a huge amount of people will never have played past the base game.
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I play on Xbox where, even though the dlc has its own achievements the percentages showing how many people have got each achievement is put of everyone who has ever played the game ever, even if they didn’t make it past the character creators I assumed PS4 was the same.