So I have a theory regarding the two chests that are only there if you kill the archers. Namely, that they're "bonus" chests. Because I accidentally missed some chest somewhere in an earlier level (I was following the guide, but maybe I died and forgot to get the chest again or something), and when I got the second-last chest, I got the achievement. And other people have reported getting the achievement earlier in the last level, when they still had 2 chests to go.
So I'm guessing that those two add to the numerator, but not the denominator, in the fraction of chests gotten.