There are a lot of games where the accuracy of your weapon is affected by whether you're looking down the sights. Meaning that even if you have a marker representing where you're reticule would be, your rounds aren't necessarily going to be landing there at medium or longer ranges. I've seen this in Killzone in particular. Borderlands and Destiny both have perks where the longer you fire a weapon the more accurate it gets, suggesting this type of dynamic aiming variable as well.