are they randomly placed in areas when you change areas?
ive had inaccessible chests at least 5 times now, most memorable for me were a floating chest in D'Alessio Seaboard and a chest in a massive tree in Ferndale :S
I think they're both random and intentionally postioned. Sort of like ANet designed multiple layouts for each map, and had the game pick out a layout at random. I don't know why some chests are ending up in unreachable places, that could probably be just the fact that someone forgot to actually check the map to test the layouts.