360 achievements listed:

i got the achievement before i got all the boxes weird enough. i did open the 4 hidden chests but im preetty sure i got the achievement before opening like the last 2-3 (non-hidden chests) you might have missed a few cuz it des add the hidden chests to the list of opened boxes so basically there are example : 21/20 boxes in the area so u might have opened a hidden one and gotten 20/20 thought thaat u got all off em but turns out there was one more.
 
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