You're browsing the GameFAQs Message Boards as a guest. Sign Up for free (or Log In if you already have an account) to be able to post messages, change how messages are displayed, and view media in posts.
So the first time I played the game I forgot one Sarcophagus (the second one in the game). I went back and played it again and got the achievement in the middle of the game. I thought it was pretty weird. I'm wondering if the game coutinued to count the Sarcophagus from the past game up to 21 and gave me the achievement!