Chronological makes sense for IG, because originally it was for mobile photography only, and meant to be posted immediately, to share with your followers where you are and what you are doing. I know a lot of people post images from trips, meals, a day in the life kind of thing, and it makes things really confusing when stuff is posted out of order. Here's my lunch, and then my breakfast, and then my dinner, and then my departure flight that left before breakfast... It's confusing when it's not chronological.